LIBPNG_LDFLAGS is never substituted...
https://bugs.gentoo.org/show_bug.cgi?id=551208

Index: pstoedit-3.70/config/pstoedit.pc.in
===================================================================
--- pstoedit-3.70.orig/config/pstoedit.pc.in
+++ pstoedit-3.70/config/pstoedit.pc.in
@@ -7,5 +7,5 @@ Name: pstoedit
 Description: converts PostScript(TM) and PDF files to other vector graphic formats
 Version: @VERSION@.0
 Requires: 
-Libs: -L@libdir@ -lpstoedit @LIBPNG_LDFLAGS@ @CXX_STD_LIB@ @CXX_RUNTIME_LIB@ @LIBLD_LDFLAGS@
+Libs: -L@libdir@ -lpstoedit @CXX_STD_LIB@ @CXX_RUNTIME_LIB@ @LIBLD_LDFLAGS@
 Cflags: -I@includedir@