Add basic Android platform module

Create a Platform/Android module that includes Platform/Linux since
Android is based on Linux.  Provide only the minimal settings needed to
get builds with Android NDK toolchains to work.

Disable use of RPATH since the Android loader ignores it and we cannot
predict the install destination anyway.

Android supports soname but shared library names must end in ".so" and
we cannot represent the versioned names with associated symlinks on all
host operating systems anyway.  However, we do want the SONAME of
library files to be set so that linking to them by path to the library
file produces NEEDED entries with the soname and not the path.  Add a
new CMAKE_PLATFORM_NO_VERSIONED_SONAME setting to tell the
cmTarget::GetLibraryNames method that not to use the VERSION or
SOVERSION target properties in the soname.
Merge topic 'android-platform' into next

42f74df6 Add basic Android platform module