glsl: automake: export abs_builddir for the tests

We're going to use them with the next commits to determine where to put
the generated tests and/or built binaries.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

src/compiler/Makefile.glsl.am

@@ -41,6 +41,7 @@ TESTS += glsl/glcpp/tests/glcpp-test.sh			\
 	glsl/tests/warnings-test.sh
 
 TESTS_ENVIRONMENT= \
+	export abs_builddir=$(abs_builddir); \
 	export PYTHON2=$(PYTHON2); \
 	export PYTHON_FLAGS=$(PYTHON_FLAGS);