Configurations/15-android.conf: add support for "standalone toolchain".

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7400)

Configurations/15-android.conf

@@ -24,7 +24,12 @@
 
             my $ndk = $ENV{ANDROID_NDK};
             die "\$ANDROID_NDK is not defined"  if (!$ndk);
-            die "\$ANDROID_NDK=$ndk is invalid" if (!-d "$ndk/platforms");
+            if (!-d "$ndk/platforms" && !-f "$ndk/AndroidVersion.txt") {
+                # $ndk/platforms is traditional "all-inclusive" NDK, while
+                # $ndk/AndroidVersion.txt is so-called standalone toolchain
+                # tailored for specific target down to API level.
+                die "\$ANDROID_NDK=$ndk is invalid";
+            }
             $ndk = canonpath($ndk);
 
             my $ndkver = undef;
@@ -40,10 +45,18 @@
                 close $fh;
             }
 
-            my $sysroot;
+            my ($sysroot, $api, $arch);
 
-            if (!($sysroot = $ENV{CROSS_SYSROOT})) {
+            $config{target} =~ m|[^-]+-([^-]+)$|;	# split on dash
-                my $api = "*";
+            $arch = $1;
+
+            if ($sysroot = $ENV{CROSS_SYSROOT}) {
+                $sysroot =~ m|/android-([0-9]+)/arch-(\w+)/?$|;
+                ($api, $arch) = ($1, $2);
+            } elsif (-f "$ndk/AndroidVersion.txt") {
+                $sysroot = "$ndk/sysroot";
+            } else {
+                $api = "*";
 
                 # see if user passed -D__ANDROID_API__=N
                 foreach (@{$useradd{CPPDEFINES}}, @{$user{CPPFLAGS}}) {
@@ -59,19 +72,15 @@
                                      } glob("$ndk/platforms/android-$api");
                 die "no $ndk/platforms/android-$api" if ($#platforms < 0);
 
-                $config{target} =~ m|[^-]+-([^-]+)$|;   # split on dash
+                $sysroot = "@platforms[$#platforms]/arch-$arch";
-                $sysroot = "@platforms[$#platforms]/arch-$1";
             }
             die "no sysroot=$sysroot"   if (!-d $sysroot);
 
-            $sysroot =~ m|/android-([0-9]+)/arch-(\w+)/?$|;
-            my ($api, $arch) = ($1, $2);
-
             my $triarch = $triplet{$arch};
             my $cflags;
             my $cppflags;
 
-            # see if there is NDK clang on $PATH
+            # see if there is NDK clang on $PATH, "universal" or "standalone"
             if (which("clang") =~ m|^$ndk/.*/prebuilt/([^/]+)/|) {
                 my $host=$1;
                 # harmonize with gcc default
@@ -83,6 +92,18 @@
                         .  "/$tritools-4.9/prebuilt/$host";
                 $user{CC} = "clang" if ($user{CC} !~ m|clang|);
                 $user{CROSS_COMPILE} = undef;
+            } elsif (-f "$ndk/AndroidVersion.txt") {    #"standalone toolchain"
+                my $cc = $user{CC} // "clang";
+                # One can probably argue that both clang and gcc should be
+                # probed, but support for "standalone toolchain" was added
+                # *after* announcement that gcc is being phased out, so
+                # favouring clang is considered adequate. Those who insist
+                # have option to enforce test for gcc with CC=gcc.
+                if (which("$triarch-$cc") !~ m|^$ndk|) {
+                    die "no NDK $triarch-$cc on \$PATH";
+                }
+                $user{CC} = $cc;
+                $user{CROSS_COMPILE} = "$triarch-";
             } elsif ($user{CC} eq "clang") {
                 die "no NDK clang on \$PATH";
             } else {

NOTES.ANDROID

@@ -24,16 +24,18 @@
  in order to invoke $(CROSS_COMPILE)gcc and company. (Configure will fail
  and give you a hint if you get it wrong.) Apart from PATH adjustment
  you need to set ANDROID_NDK environment to point at NDK directory
- as /some/where/android-ndk-<ver>. NDK customarily supports multiple
+ as /some/where/android-ndk-<ver>. Both variables are significant at both
+ configuration and compilation times. NDK customarily supports multiple
  Android API levels, e.g. android-14, android-21, etc. By default latest 
  one available is chosen. If you need to target older platform, pass
  additional -D__ANDROID_API__=N to Configure. N is numeric value of the
  target platform version. For example, to compile for ICS on ARM with
  NDK 10d:
 
-    ANDROID_NDK=/some/where/android-ndk-10d
+    export ANDROID_NDK=/some/where/android-ndk-10d
     PATH=$ANDROID_NDK/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin:$PATH
     ./Configure android-arm -D__ANDROID_API__=14
+    make
 
  Caveat lector! Earlier OpenSSL versions relied on additional CROSS_SYSROOT
  variable set to $ANDROID_NDK/platforms/android-<api>/arch-<arch> to
@@ -43,12 +45,18 @@
  conflict, and mixing the two is therefore not supported. Migration to
  CROSS_SYSROOT-less setup is recommended.
 
- One can engage clang by adjusting PATH to cover NDK's clang. Just keep
+ One can engage clang by adjusting PATH to cover same NDK's clang. Just
- in mind that if you miss it, Configure will try to use gcc... Also,
+ keep in mind that if you miss it, Configure will try to use gcc...
- PATH would need even further adjustment to cover unprefixed, yet
+ Also, PATH would need even further adjustment to cover unprefixed, yet
  target-specific, ar and ranlib. It's possible that you don't need to
  bother, if binutils-multiarch is installed on your Linux system.
 
+ Another option is to create so called "standalone toolchain" tailored
+ for single specific platform including Android API level, and assign its
+ location to ANDROID_NDK. In such case you have to pass matching target
+ name to Configure and shouldn't use -D__ANDROID_API__=N. PATH adjusment
+ becomes simpler, $ANDROID_NDK/bin:$PATH suffices.
+
  Running tests (on Linux)
  ------------------------