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>