Fix Android target build on Apple hosts.

Currently CMakeLists.txt forces CMAKE_C_COMPILER to clang whenever
compiling on an APPLE host.  Unfortunately, this prevents the
android.toolchain.cmake script from selecting the correct cross
compiler. See [line 1136][].

To avoid this problem, only force clang if ANDROID_NDK is not set.  This
is one of the variables used by Android.mk.

[line 1136]: https://github.com/apitrace/apitrace/blob/master/cmake/toolchain/android.toolchain.cmake#L1136
Define _GNU_SOURCE for the whole tree.

As not all dlfnc.h includes define it consistently.
Implement interposing of dlopen on Android

Consolidate all implementations of unredirected dlopen in one place.

Use unredirected dlopen is glproc_egl.cpp as done in glproc_gl.cpp.

Check that tracing is enabled before redirecting dlopen.

Signed-off-by: José Fonseca <jose.r.fonseca@gmail.com>