libmirage: fix gobject-introspection files installation

See patch comment.

srcpkgs/libmirage/patches/girpath.patch

@@ -0,0 +1,24 @@
+The girdir in the .pc file is defined like this:
+
+${pc_sysrootdir}/${datadir}/gir-1.0
+
+If you replace pc_sysrootdir with a /, and datadir with /usr/share, you end
+up with ///usr/share/gir-1.0, which cmake treats as a network path, and
+breaks when used with destdir
+
+So replace a useless substitute (we don't use a non-standard prefix) with
+a useful one
+
+--- cmake/GObjectIntrospection.cmake	2019-02-11 02:04:33.574070141 +0100
++++ cmake/GObjectIntrospection.cmake	2019-02-11 02:05:26.556485411 +0100
+@@ -44,8 +44,8 @@
+ _pkg_config_variable(girdir GIR_GIRDIR)
+ _pkg_config_variable(typelibdir GIR_TYPELIBDIR)
+ 
+-string (REPLACE "/usr" "${CMAKE_INSTALL_PREFIX}" GIR_GIRDIR "${GIR_GIRDIR}")
+-string (REPLACE "/usr" "${CMAKE_INSTALL_PREFIX}" GIR_TYPELIBDIR "${GIR_TYPELIBDIR}")
++string (REPLACE "///" "/" GIR_GIRDIR "${GIR_GIRDIR}")
++string (REPLACE "///" "/" GIR_TYPELIBDIR "${GIR_TYPELIBDIR}")
+ 
+ ###############################################################################
+ # The main function