build: use dependency for matrix.c

matrix.c needs to be built differently for a test program vs. everything else,
so it cannot be in a helper lib. Instead, make a dependency object for it for
easy use which always gets all the paths correct automatically.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

shared/meson.build

@@ -76,3 +76,9 @@ dep_lib_cairo_shared = declare_dependency(
 	link_with: lib_cairo_shared,
 	dependencies: deps_cairo_shared
 )
+
+dep_matrix_c = declare_dependency(
+	sources: 'matrix.c',
+	include_directories: public_inc,
+	dependencies: dep_libm
+)