refactor -lgd in @LIBS

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
see https://github.com/lstein/Perl-GD/pull/7

Index: b/Makefile.PL
===================================================================
--- a/Makefile.PL
+++ b/Makefile.PL
@@ -34,9 +34,9 @@
 END
 }
 
+push @LIBS, "-lgd";
 @INC     = qw(-I/usr/include -I/usr/include/gd) unless @INC;
 @LIBPATH = qw(-L/usr/lib/X11 -L/usr/X11R6/lib -L/usr/X11/lib -L/usr/lib) unless @LIBPATH;
-@LIBS    = qw(-lgd) unless @LIBS;
 
 # support for AMD64 libraries
 if (-d '/usr/lib64') {
@@ -291,7 +291,6 @@
   @$LIBPATH      = map {s/^-L// && "-L$_"} split /\s+/,$ldflags;
   @$LIBS         = split /\s+/,$libs;
 
-  push @$LIBS,"-lgd";
   push @$LIBPATH,"-L$libdir";
   ($$lib_gd_path = $libdir) =~ s!/[^/]+$!!;
   $$options      = $features;