Instead of tweaking the linker flags ourselves. This allows Meson to do
the right thing, and pass the arguments only where needed.

Fix return value of shading language for GLES2.

Throughout the mesa project we've been using 100 for GLES2's shading
language.  It was pretty clearly the intent here, but the clever
inline detection of "am I parsing a GLSL version or a GL version
string" forgot about GLSL 1.0.x, and thus returned 10.

Add call convention to mock function

Use EGL_NO_X11 to disable X11 headers

MESA_EGL_NO_X11_HEADERS has been deprecated, and libglvnd only supports
EGL_NO_X11.

Fixes: #214

build: Use find_program()

Otherwise installing Python3 will break horribly.