configure: Use KRB5CONFIG for krb5-config

Allows the user to easier override its path.

Bug: http://curl.haxx.se/bug/view.cgi?id=1486

configure.ac

@@ -1184,6 +1184,8 @@ AC_ARG_WITH(gssapi,
   fi
 ])
 
+: ${KRB5CONFIG:="$GSSAPI_ROOT/bin/krb5-config"}
+
 save_CPPFLAGS="$CPPFLAGS"
 AC_MSG_CHECKING([if GSS-API support is requested])
 if test x"$want_gss" = xyes; then
@@ -1194,6 +1196,8 @@ if test x"$want_gss" = xyes; then
         GSSAPI_INCS=`$GSSAPI_ROOT/bin/$host_alias-krb5-config --cflags gssapi`
      elif test -f "$GSSAPI_ROOT/bin/krb5-config"; then
         GSSAPI_INCS=`$GSSAPI_ROOT/bin/krb5-config --cflags gssapi`
+     elif test -f "$KRB5CONFIG"; then
+        GSSAPI_INCS=`$KRB5CONFIG --cflags gssapi`
      elif test "$GSSAPI_ROOT" != "yes"; then
         GSSAPI_INCS="-I$GSSAPI_ROOT/include"
      fi
@@ -1283,10 +1287,10 @@ if test x"$want_gss" = xyes; then
            dnl into LIBS
            gss_libs=`$GSSAPI_ROOT/bin/$host_alias-krb5-config --libs gssapi`
            LIBS="$gss_libs $LIBS"
-        elif test -f "$GSSAPI_ROOT/bin/krb5-config"; then
+        elif test -f "$KRB5CONFIG"; then
            dnl krb5-config doesn't have --libs-only-L or similar, put everything
            dnl into LIBS
-           gss_libs=`$GSSAPI_ROOT/bin/krb5-config --libs gssapi`
+           gss_libs=`$KRB5CONFIG --libs gssapi`
            LIBS="$gss_libs $LIBS"
         else
            case $host in