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+
+ NOTES FOR ANDROID PLATFORMS
+ ===========================
+
+ Requirement details
+ -------------------
+
+ Beside basic tools like perl and make you'll need to download the Android
+ NDK. It's available for Linux, macOS and Windows, but only Linux
+ version was actually tested. There is no reason to believe that macOS
+ wouldn't work. And as for Windows, it's unclear which "shell" would be
+ suitable, MSYS2 might have best chances. NDK version should play lesser
+ role, the goal is to support a range of most recent versions.
+
+ Configuration
+ -------------
+
+ Android is a cross-compiled target and you can't rely on ./Configure
+ to find out the configuration target for you.  You have to name your
+ target explicitly; there are android-arm, android-arm64, android-mips,
+ android-mip64, android-x86 and android-x86_64 (*MIPS targets are no
+ longer supported with NDK R20+).
+
+ Do not pass --cross-compile-prefix (as you might be tempted), as it
+ will be "calculated" automatically based on chosen platform. However,
+ you still need to know the prefix to extend your PATH, in order to
+ invoke $(CROSS_COMPILE)clang [*gcc on NDK 19 and lower] and company.
+ (Configure will fail and give you a hint if you get it wrong.)
+
+ Apart from PATH adjustment you need to set ANDROID_NDK_ROOT environment
+ to point at the NDK directory. If you're using a side-by-side NDK the path
+ will look something like /some/where/android-sdk/ndk/<ver>, and for a
+ standalone NDK the path will be something like /some/where/android-ndk-<ver>.
+ Both variables are significant at both configuration and compilation times.
+ The NDK customarily supports multiple Android API levels, e.g. android-14,
+ android-21, etc. By default latest API level is chosen. If you need to
+ target an older platform pass the argument -D__ANDROID_API__=N to Configure,
+ with N being the numerical value of the target platform version. For example,
+ to compile for Android 10 arm64 with a side-by-side NDK r20.0.5594570
+
+    export ANDROID_NDK_ROOT=/home/whoever/Android/android-sdk/ndk/20.0.5594570
+    PATH=$ANDROID_NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64/bin:$ANDROID_NDK_ROOT/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin:$PATH
+    ./Configure android-arm64 -D__ANDROID_API__=29
+    make
+
+ Older versions of the NDK have GCC under their common prebuilt tools
+ directory, so the bin path will be slightly different. EG: to compile
+ for ICS on ARM with NDK 10d:
+
+    export ANDROID_NDK_ROOT=/some/where/android-ndk-10d
+    PATH=$ANDROID_NDK_ROOT/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin:$PATH
+    ./Configure android-arm -D__ANDROID_API__=14
+    make
+
+ Caveat lector! Earlier OpenSSL versions relied on additional CROSS_SYSROOT
+ variable set to $ANDROID_NDK_ROOT/platforms/android-<api>/arch-<arch> to
+ appoint headers-n-libraries' location. It's still recognized in order
+ to facilitate migration from older projects. However, since API level
+ appears in CROSS_SYSROOT value, passing -D__ANDROID_API__=N can be in
+ 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 same NDK's clang. Just
+ keep in mind that if you miss it, Configure will try to use gcc...
+ 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_ROOT. In such case you have to pass matching
+ target name to Configure and shouldn't use -D__ANDROID_API__=N. PATH
+ adjustment becomes simpler, $ANDROID_NDK_ROOT/bin:$PATH suffices.
+
+ Running tests (on Linux)
+ ------------------------
+
+ This is not actually supported. Notes are meant rather as inspiration.
+
+ Even though build output targets alien system, it's possible to execute
+ test suite on Linux system by employing qemu-user. The trick is static
+ linking. Pass -static to Configure, then edit generated Makefile and
+ remove occurrences of -ldl and -pie flags. You would also need to pick
+ API version that comes with usable static libraries, 42/2=21 used to
+ work. Once built, you should be able to
+
+    env EXE_SHELL=qemu-<arch> make test
+
+ If you need to pass additional flag to qemu, quotes are your friend, e.g.
+
+    env EXE_SHELL="qemu-mips64el -cpu MIPS64R6-generic" make test