test_pkgconfig_gen_deps: set PKG_CONFIG_SYSTEM_LIBRARY_PATH=/usr/lib

pkgconf automatically prunes "system library paths" from its output. The
system library paths depend on the system toolchain. A common value on a
64-bit system is as follows:

/lib64:/usr/lib64:/usr/local/lib64

So, if -L/usr/lib64 appears in the Libs section, it will be pruned from
the output of pkg-config --libs.

The pc files generated for this test contain something like this:

libdir=/usr/lib
Libs: -L${libdir} ...

pkgconf may not consider /usr/lib to be a system library path, so it is
not pruned as the test expects. To work around this, override the
compiled-in list of paths via the PKG_CONFIG_SYSTEM_LIBRARY_PATH
environment variable.

Fixes: https://github.com/mesonbuild/meson/issues/6004

run_unittests.py

@@ -4899,7 +4899,10 @@ class LinuxlikeTests(BasePlatformTests):
         privatedir2 = self.privatedir
 
         os.environ
-        env = {'PKG_CONFIG_LIBDIR': os.pathsep.join([privatedir1, privatedir2])}
+        env = {
+            'PKG_CONFIG_LIBDIR': os.pathsep.join([privatedir1, privatedir2]),
+            'PKG_CONFIG_SYSTEM_LIBRARY_PATH': '/usr/lib',
+        }
         self._run(['pkg-config', 'dependency-test', '--validate'], override_envvars=env)
 
         # pkg-config strips some duplicated flags so we have to parse the