build: Fix mingw ssl gdi32 order

- If mingw ssl make sure -lgdi32 comes after ssl libs

- Allow PKG_CONFIG to set pkg-config location and options

Bug: https://github.com/bagder/curl/pull/501
Reported-by: Kang Lin

acinclude.m4

@@ -3052,9 +3052,17 @@ dnl Optionally PKG_CONFIG_LIBDIR may be given as $pcdir.
 dnl
 
 AC_DEFUN([CURL_CHECK_PKGCONFIG], [
-    AC_PATH_TOOL( PKGCONFIG, pkg-config, no, $PATH:/usr/bin:/usr/local/bin)
 
-    if test x$PKGCONFIG != xno; then
+    if test -z "$PKGCONFIG"; then
+      PKGCONFIG="$PKG_CONFIG"
+    fi
+
+    if test -z "$PKGCONFIG"; then
+      AC_PATH_TOOL([PKGCONFIG], [pkg-config], [no],
+        [$PATH:/usr/bin:/usr/local/bin])
+    fi
+
+    if test "x$PKGCONFIG" != "xno"; then
       AC_MSG_CHECKING([for $1 options with pkg-config])
       dnl ask pkg-config about $1
       itexists=`CURL_EXPORT_PCDIR([$2]) dnl