Force autoconf to lookup the macro

Fix based on discussion with @ankon on this subject. This problem is not trivial to reproduce as different people have encountered this since the explicit failure added in 339ac72 (but first we didn't fail on not resolving the macro).

@meisterT found the proper documentation on this see: https://manpages.debian.org/testing/pkgconf/pkg.m4.7.en.html

configure.ac

@@ -217,7 +217,7 @@ AX_WITH_COMMENT(7,[      ])
 
 # {{{ Directory for systemd unit files
 
-PKG_PROG_PKG_CONFIG
+PKG_PROG_PKG_CONFIG()
 AC_ARG_WITH([systemdsystemunitdir],
      [AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files])],,
      [with_systemdsystemunitdir=auto])