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 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.


 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
	./Configure android-arm64 -D__ANDROID_API__=29

 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
    ./Configure android-arm -D__ANDROID_API__=14

 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