make: Don't use builtin_stubs.cpp for standalone GLSL compiler.

make: Don't use builtin_stubs.cpp for standalone GLSL compiler.

builtin_stubs.cpp is only supposed to be used for builtin_compiler.  It
contains a stub version of _mesa_glsl_initialize_functions() that does
nothing.

libglsl.a already contains builtin_function.cpp, the generated file that
contains a version of _mesa_glsl_initialize_functions() that actually
initializes all the built-in functions.

By mistakenly linking to builtin_stubs, glsl_compiler and glsl_test are
unable to compile any shaders that use built-in functions.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>