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_CONFIGAC_LANG_CPLUSPLUS