Adjust to automake 1.16.4 changes

--- a/compizconfig-python.pc.in	2020-04-04 20:41:47.000000000 +0200
+++ b/compizconfig-python.pc.in	2021-10-17 10:02:14.836742131 +0200
@@ -2,6 +2,7 @@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
+PYTHON_EXEC_PREFIX=@PYTHON_EXEC_PREFIX@
 pyexecdir=@pyexecdir@
 
 Name: compizconfig-python
