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

--- a/cmake/GObjectIntrospection.cmake	2019-02-11 02:04:33.574070141 +0100
+++ b/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