Fix usage of gdlib-config

gnuplot configure.in script properly takes care of finding
gdlib-config using AC_PATH_PROG... but then directly uses gdlib-config
instead of going through the GDLIB_CONFIG variable that AC_PATH_PROG
has defined. Which means that whenever a gdlib-config binary not in
the PATH is being used, it does not use it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/configure.in
===================================================================
--- a/configure.in
+++ b/configure.in
@@ -537,9 +537,9 @@
 if test "$with_gd" != no; then
   AC_PATH_PROG([GDLIB_CONFIG], [gdlib-config])
   if test -n "$GDLIB_CONFIG"; then
-    libgd_CPPFLAGS=`gdlib-config --cflags`
-    libgd_LDFLAGS=`gdlib-config --ldflags`
-    libgd_LIBS=`gdlib-config --libs`
+    libgd_CPPFLAGS=`$GDLIB_CONFIG --cflags`
+    libgd_LDFLAGS=`$GDLIB_CONFIG --ldflags`
+    libgd_LIBS=`$GDLIB_CONFIG --libs`
   elif test -d "$with_gd"; then
     libgd_CPPFLAGS="-I$with_gd/include"
     libgd_LDFLAGS="-L$with_gd/lib"