Use pkg-config to report libpng version and installation directories.

diff -Naur libpng-1.5.5.orig/libpng-config.in libpng-1.5.5/libpng-config.in

--- libpng-1.5.5.orig/libpng-config.in	2011-09-22 09:40:23.000000000 -0400

+++ libpng-1.5.5/libpng-config.in	2011-10-05 01:03:32.335435187 -0400

@@ -11,11 +11,11 @@

 # Modeled after libxml-config.

-version="@PNGLIB_VERSION@"

-prefix="@prefix@"

-exec_prefix="@exec_prefix@"

-libdir="@libdir@"

-includedir="@includedir@/libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@"

+version=`pkg-config --modversion libpng`

+prefix=`pkg-config --variable prefix libpng`

+exec_prefix=`pkg-config --variable exec_prefix libpng`

+libdir=`pkg-config --variable libdir libpng`

+includedir=`pkg-config --variable includedir libpng`

 libs="-lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@"

 all_libs="-lpng@PNGLIB_MAJOR@@PNGLIB_MINOR@ @LIBS@"

 I_opts="-I${includedir}"