[devel] Moved libpng-config.in and libpng.pc-configure.in out of the scripts

directory, to libpng-config.in and libpng-pc.in, respectively, and
    modified Makefile.am and configure.ac accordingly.  Now "configure"
    needs nothing from the "scripts" directory.
  Also, avoid redefining PNG_CONST in pngconf.h

Makefile.am

@@ -73,7 +73,6 @@ EXTRA_DIST= \
 	${srcdir}/projects/visualc6/* \
 	${srcdir}/projects/visualc71/* \
 	${srcdir}/projects/xcode/* \
-	${srcdir}/scripts/* \
 	${srcdir}/contrib/gregbook/* \
 	${srcdir}/contrib/pngminim/* \
 	${srcdir}/contrib/pngminus/* \