We had removed PKG_PROG_PKG_CONFIG
in 21b2ebaf74222017f85123deb6f30a33c7678513
(#1090). But then then the not rebased (!) merge of 2be6ba0fedd0d2d62ba6f346d7ced7abde0d66e4
(#1084) brought that macro back at another location, without git
complaining about a conflict.
Fixes #1127.