Simplify building with the Android NDK.

Simplify building with the Android NDK.

Check --host against *-*-linux-android* for Android-specific configure fixes,
and improve **/Makefile.am to simplify Android support:

	./configure \
		--build=`./config.guess` \
		--host=armv5-linux-androideabi \
		--target=armv5-linux-androideabi \
		--enable-nls=no --with-mcs-docs=no --enable-mcs-build=no \
		CC="/path/to/ndk/arm-linux-androideabi-gcc --sysroot=/path/to/sysroot" \
		AR="..." AS="..." CPP="..." CXX="..." LD="..." RANLIB="..." STRIP="..."
		CFLAGS="-DARM_FPU_NONE=1" CXXFLAGS="-DARM_FPU_NONE=1"

See also Android NDK docs/STANDALONE-TOOLCHAIN.html.

The Android NDK/bionic is "interesting", in that it's lacking header files and
macros normally present on Linux which otherwise break the build (e.g. no
<link.h> which breaks Boehm support).  Using --host with an android target
works around the known issues, simplifying the building of libmonosgen-2.0.so
with the Android NDK toolchain.