Fix test_static_link() on MacOSX

test3-static was actually always using the shared library because that
warning was not fatal:
  WARNING: Static library 'func6' not found for dependency 'func6', may
  not be statically linked

The reason why the libfunc6.a wasn't found is because the prefix in the
generated pc file was not set to install dir.

run_unittests.py

@@ -5642,19 +5642,22 @@ c = ['{0}']
 
         # Build some libraries and install them
         testdir = os.path.join(self.unit_test_dir, '69 static link/lib')
-        libdir = os.path.join(self.installdir, self.prefix[1:], self.libdir)
+        libdir = os.path.join(self.installdir, self.libdir)
+        oldprefix = self.prefix
+        self.prefix = self.installdir
         self.init(testdir)
-        self.install()
+        self.install(use_destdir=False)
 
         # Test that installed libraries works
         self.new_builddir()
+        self.prefix = oldprefix
+        meson_args = ['-Dc_link_args=-L{}'.format(libdir),
+                      '--fatal-meson-warnings']
         testdir = os.path.join(self.unit_test_dir, '69 static link')
         env = {'PKG_CONFIG_LIBDIR': os.path.join(libdir, 'pkgconfig')}
-        run_env = {'LD_LIBRARY_PATH': libdir}
-        self.init(testdir, extra_args=['-Dc_link_args="-L{}"'.format(libdir)],
-                  override_envvars=env)
+        self.init(testdir, extra_args=meson_args, override_envvars=env)
         self.build()
-        self.run_tests(override_envvars=run_env)
+        self.run_tests()
 
 def should_run_cross_arm_tests():
     return shutil.which('arm-linux-gnueabihf-gcc') and not platform.machine().lower().startswith('arm')