build: Fix Qt5PlatformSupport check without pkg-config

The non-pkg-config case can't use pkg-config to check the version.

Also, make sure that the check is properly guarded in the case of missing
pkg-config macros.

build-aux/m4/bitcoin_qt.m4

@@ -331,8 +331,9 @@ AC_DEFUN([_BITCOIN_QT_FIND_STATIC_PLUGINS],[
           QT_LIBS="$QT_LIBS -L$qt_plugin_path/accessible"
         fi
       fi
-     m4_ifdef([PKG_CHECK_MODULES],[
      if test x$use_pkgconfig = xyes; then
+     : dnl
+     m4_ifdef([PKG_CHECK_MODULES],[
        PKG_CHECK_MODULES([QTPLATFORM], [Qt5PlatformSupport], [QT_LIBS="$QTPLATFORM_LIBS $QT_LIBS"])
        if test x$TARGET_OS = xlinux; then
          PKG_CHECK_MODULES([X11XCB], [x11-xcb], [QT_LIBS="$X11XCB_LIBS $QT_LIBS"])
@@ -342,12 +343,23 @@ AC_DEFUN([_BITCOIN_QT_FIND_STATIC_PLUGINS],[
        elif test x$TARGET_OS = xdarwin; then
          PKG_CHECK_MODULES([QTPRINT], [Qt5PrintSupport], [QT_LIBS="$QTPRINT_LIBS $QT_LIBS"])
        fi
+     ])
      else
-       if ${PKG_CONFIG} --exists "Qt5Core >= 5.6" 2>/dev/null; then
-         QT_LIBS="-lQt5PlatformSupport $QT_LIBS"
+       if test x$TARGET_OS = xwindows; then
+         AC_CACHE_CHECK(for Qt >= 5.6, bitcoin_cv_need_platformsupport,[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+             [[#include <QtCore>]],[[
+             #if QT_VERSION < 0x050600
+             choke;
+             #endif
+             ]])],
+           [bitcoin_cv_need_platformsupport=yes],
+           [bitcoin_cv_need_platformsupport=no])
+         ])
+         if test x$bitcoin_cv_need_platformsupport = xyes; then
+           BITCOIN_QT_CHECK(AC_CHECK_LIB([${QT_LIB_PREFIX}PlatformSupport],[main],,BITCOIN_QT_FAIL(lib$QT_LIB_PREFIXPlatformSupport not found)))
+         fi
        fi
      fi
-     ])
   else
     if test x$qt_plugin_path != x; then
       QT_LIBS="$QT_LIBS -L$qt_plugin_path/accessible"