pkgconfig: set libdir only if used

This should address some concerns outlined in
https://github.com/yhirose/cpp-httplib/pull/1182

mesonbuild/modules/pkgconfig.py

@@ -347,6 +347,7 @@ class PkgConfigModule(ExtensionModule):
                 ofile.write('prefix={}\n'.format(self._escape(prefix)))
                 if uninstalled:
                     ofile.write('srcdir={}\n'.format(self._escape(srcdir)))
+                if deps.pub_libs or deps.priv_libs:
                     ofile.write('libdir={}\n'.format(self._escape('${prefix}' / libdir)))
                 ofile.write('includedir={}\n'.format(self._escape('${prefix}' / incdir)))
             if variables or unescaped_variables: