Allows to properly detect libpng if USE="-truetype"

https://bugs.gentoo.org/show_bug.cgi?id=326803

Patch written by Carlo Marcelo Arenas Belon <carenas@sajinet.com.pe>
--- configure.ac
+++ configure.ac
@@ -10,6 +10,7 @@
 AC_PROG_CXX
 AC_PROG_INSTALL
 AC_PROG_SED
+PKG_PROG_PKG_CONFIG
 
 AC_LANG_CPLUSPLUS