autotools: fix compiling/linking gl engines on OSX.

autotools: fix compiling/linking gl engines on OSX.

Summary:
Until now, it was necessary to set global LDFLAGS and CFLAGS to allow
compiling (and linking) engines using OpenGL.
gl_generic used to complained about missing headers or unkown libraries.
A problem on OSX is that there is CGL (Apple's Core OpenGL) on which the whole system
relies on and GLX, when X11 is installed; and they cohabit together.
When gl_cocoa is enabled, gl_generic is now linked against CGL.
When it is not, gl_generic is compiled with and linked against GLX as a fallback.

@fix

Test Plan:
With --enable-cocoa: software_x11, opengl_x11 and opengl_cocoa are working as expected.
With --disable-cocoa: software_x11 and opengl_x11 are also working as expected.
No compiling nor linking problems have been issued.

Reviewers: cedric, raster, raoulh

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D1723

Signed-off-by: Cedric BAIL <cedric@osg.samsung.com>