./configure failed to treat libpng's '--libs'

Hi,

I have found the recent FreeBSD port of libpng has libpng12-config,
which works well, but in fact is a symlink of libpng-config;

> $ libpng12-config --libs
> -lpng -lz -lm

Thus ./configure come from configure.ac in CVS head fails to
treat the expected option '-lpng12', like that;

> $ ./configure --with-png=/usr/local
> checking build system type... i386-unknown-freebsd6.3
> checking host system type... i386-unknown-freebsd6.3
> checking target system type... i386-unknown-freebsd6.3
> checking for a BSD-compatible install... /usr/bin/install -c
> checking whether build environment is sane... yes
> checking for a thread-safe mkdir -p... config/install-sh -c -d
> (snip)
> checking for sin... no
> checking for sin in -lm... yes
> checking for deflate in -lz... yes
> checking png.h usability... yes
> checking png.h presence... yes
> checking for png.h... yes
> checking for png_create_read_struct in -lpng12... no
> configure: WARNING: libpng is required - see http://www.libpng.org/pub/png/
> checking for freetype-config... /usr/local/bin/freetype-config
> checking for FT_Init_FreeType in -lfreetype... yes
> checking ft2build.h usability... yes
> checking ft2build.h presence... yes
> checking for ft2build.h... yes
> checking for FcInit in -lfontconfig... yes
> checking for jpeg_set_defaults in -ljpeg... yes
> checking for XpmReadFileToXpmImage in -lXpm... yes
> checking for the pthreads library -lpthreads... no
> checking whether pthreads work without any flags... no
> checking whether pthreads work with -Kthread... no
> checking whether pthreads work with -kthread... no
> checking for the pthreads library -llthread... no
> checking whether pthreads work with -pthread... yes
> checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
> checking if more special flags are required for pthreads... -D_THREAD_SAFE

> ** Configuration summary for gd 2.1.0:

>    Support for PNG library:
>    Support for JPEG library:         yes
>    Support for Freetype 2.x library: yes
>    Support for Fontconfig library:   yes
>    Support for Xpm library:          yes
>    Support for pthreads:             yes


The attached patch is a workaround for the behavior,
and any suggestion?

Thanks,
-- Takeshi Abe

Index: libgd/src/configure.ac
===================================================================
RCS file: /repository/gd/libgd/src/configure.ac,v
retrieving revision 1.36
diff -u -r1.36 configure.ac
--- libgd/src/configure.ac	30 Jan 2008 11:21:15 -0000	1.36
+++ libgd/src/configure.ac	12 Jun 2008 06:30:49 -0000
@@ -424,33 +424,26 @@
       [LIBS="-lpng12 $LIBS"
        FEATURES="GD_PNG $FEATURES"
        AC_DEFINE(HAVE_LIBPNG, 1, [ Define if you have the png library. ])],
-      [LDFLAGS="$_ldflags"  
-       AC_MSG_WARN([libpng is required - see http://www.libpng.org/pub/png/])])
-  elif test -n "$LIBPNG_CONFIG"; then
-    AC_CHECK_LIB(png,png_create_read_struct,
-      [LIBS="-lpng $LIBS" 
-       FEATURES="GD_PNG $FEATURES"
-       AC_DEFINE(HAVE_LIBPNG, 1, [ Define if you have the png library. ])],
-      [LDFLAGS="$_ldflags"  
-       AC_MSG_WARN([libpng is required - see http://www.libpng.org/pub/png/])])
+      [LDFLAGS="$_ldflags"])
+  fi
+  #TBB: png12's png_create_read_struct is just as good 
+  #2.0.23: should be = not ==  
+  if test "$ac_cv_lib_png12_png_create_read_struct" = yes; then
+    ac_cv_lib_png_png_create_read_struct=yes
   else
     AC_CHECK_LIB(png,png_create_read_struct,
       [LIBS="-lpng $LIBS" 
        FEATURES="GD_PNG $FEATURES"
        AC_DEFINE(HAVE_LIBPNG, 1, [ Define if you have the png library. ])],
-      [LDFLAGS="$_ldflags"  
-       AC_MSG_WARN([libpng is required - see http://www.libpng.org/pub/png/])])
+      [LDFLAGS="$_ldflags"])
   fi
 else
   ac_cv_lib_png_png_create_read_struct=no
-  AC_MSG_WARN([libpng is required - see http://www.libpng.org/pub/png/])
 fi
 
-#TBB: png12's png_create_read_struct is just as good 
-#2.0.23: should be = not ==  
-if test "$ac_cv_lib_png12_png_create_read_struct" = yes; then
-  ac_cv_lib_png_png_create_read_struct=yes
-fi 
+if test "$ac_cv_lib_png_png_create_read_struct" != yes; then
+  AC_MSG_WARN([libpng is required - see http://www.libpng.org/pub/png/])
+fi
 
 dnl FreeType configure tests snarfed from libwmf ..
 AC_ARG_WITH(freetype,dnl