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, Mac OS X and Windows, but only Linux

 version was actually tested. There is no reason to believe that Mac OS X

 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 naturally cross-compiled target and you can't use ./config.

 You have to use ./Configure and 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. Though 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_HOME 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_HOME=/home/whoever/Android/android-sdk/ndk/20.0.5594570

	PATH=$ANDROID_NDK_HOME/toolchains/llvm/prebuilt/linux-x86_64/bin:$ANDROID_NDK_HOME/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_HOME=/some/where/android-ndk-10d

    PATH=$ANDROID_NDK_HOME/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin:$PATH

    ./Configure android-arm -D__ANDROID_API__=14

    make

 Caveat lector! Earlier OpenSSL versions relied on additional CROSS_SYSROOT

 variable set to $ANDROID_NDK_HOME/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_HOME. In such case you have to pass matching

 target name to Configure and shouldn't use -D__ANDROID_API__=N. PATH

 adjustment becomes simpler, $ANDROID_NDK_HOME/bin:$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