make with-android-headers optional

If with-android-headers is not specified, try to find it
with pkg-config.

Then, extract Android versions from android-version.h but take
ANDROID_HEADERS_CFLAGS into account.

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>
replace -I/include/android with ANDROID_HEADERS_CFLAGS

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>
hwcomposer-egl.pc: link with hybris-hardware

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>