pkgconfig: Handle prefix in library path

The install_dir parameter of the libraries can also contain the
prefix path, which creates wrong library paths in the .pc file.

This patch detects if prefix is contained in the library path
and creates a relative path.

Fixes #2469

mesonbuild/modules/pkgconfig.py

@@ -55,6 +55,15 @@ class PkgConfigModule(ExtensionModule):
             value = value.as_posix()
         return value.replace(' ', '\ ')
 
+    def _make_relative(self, prefix, subdir):
+        if isinstance(prefix, PurePath):
+            prefix = prefix.as_posix()
+        if isinstance(subdir, PurePath):
+            subdir = subdir.as_posix()
+        if subdir.startswith(prefix):
+            subdir = subdir.replace(prefix, '')
+        return subdir
+
     def generate_pkgconfig_file(self, state, libraries, subdirs, name, description,
                                 url, version, pcfile, pub_reqs, priv_reqs,
                                 conflicts, priv_libs, extra_cflags, variables):
@@ -98,7 +107,7 @@ class PkgConfigModule(ExtensionModule):
                         if install_dir is False:
                             continue
                         if isinstance(install_dir, str):
-                            yield '-L${prefix}/%s ' % self._escape(install_dir)
+                            yield '-L${prefix}/%s ' % self._escape(self._make_relative(prefix, install_dir))
                         else:  # install_dir is True
                             yield '-L${libdir}'
                         lname = self._get_lname(l, msg, pcfile)