do not try to recongnise libpng without pkg-config

Author: dimpase

build/pkgs/libpng/spkg-configure.m4

@@ -1,15 +1,8 @@
 SAGE_SPKG_CONFIGURE([libpng], [
     m4_pushdef([SAGE_LIBPNG_MINVER],[1.2])
     SAGE_SPKG_DEPCHECK([zlib], [
-      dnl First try checking for libpng with pkg-config
-      PKG_CHECK_MODULES([LIBPNG], [libpng >= $SAGE_LIBPNG_MINVER], [], [
-        dnl Fallback to manually grubbing around for headers and libs
-        AC_CHECK_HEADERS([png.h], [
-           AC_SEARCH_LIBS([png_get_io_ptr], [png], [
-             dnl write out libpng.pc
-           ], [sage_spkg_install_libpng=yes])
-         ], [sage_spkg_install_libpng=yes])
-      ])
+      PKG_CHECK_MODULES([LIBPNG], [libpng >= $SAGE_LIBPNG_MINVER], [
+      ], [sage_spkg_install_libpng=yes])
     ])
     m4_popdef([SAGE_LIBPNG_MINVER])
 ])