Revert "configure: Abort if Xlib isn't present when building for XCB."

the change is wrong for multiple reasons:
- it is possible to build qt (the offscreen plugin) with xlib but
  without xcb, which was impossible after the change
- the check was built in a way that if xcb is auto-detected but xlib is
  missing, configure would abort (instead of disabling xcb), which is
  unreasonable (cf. linked task)
- it should be possible to build the xcb plugin without xlib - it's
  testing for xcb-xlib and should have the correct #ifdefs. if these are
  broken, they should be fixed. if this is unrealistic, the build system
  should be adjusted to this fact ... differently.

This reverts commit 683451e7c7.

Task-number: QTBUG-34382
Change-Id: I1c5811e2fb1f09b084b6aeb7350e5f376e1b9b82
Reviewed-by: Nicolás Alvarez <nicolas.alvarez@gmail.com>
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

configure

@@ -5080,6 +5080,11 @@ if [ "$CFG_TSLIB" != "no" ]; then
     fi
 fi
 
+# Check we actually have X11 :-)
+if compileTest x11/xlib "XLib"; then
+    QT_CONFIG="$QT_CONFIG xlib"
+fi
+
 if [ "$CFG_XCB" != "no" ]; then
     if [ -n "$PKG_CONFIG" ] && $PKG_CONFIG --exists "xcb >= 1.5" 2>/dev/null; then
         QMAKE_CFLAGS_XCB="`$PKG_CONFIG --cflags xcb 2>/dev/null`"
@@ -5125,17 +5130,6 @@ if [ "$CFG_XCB" != "no" ]; then
             fi
         fi
 
-        # Check for X11. Eventually we should port everything to XCB,
-        # but for now the port is incomplete and Xlib is a requirement.
-        if compileTest x11/xlib "XLib"; then
-            QT_CONFIG="$QT_CONFIG xlib"
-        else
-            echo "The test for linking against Xlib failed!"
-            echo " You might need to install dependency packages."
-            echo " See src/plugins/platforms/xcb/README."
-            exit 1
-        fi
-
         if compileTest qpa/xcb-xlib "xcb-xlib" $QMAKE_CFLAGS_XCB $QMAKE_LIBS_XCB; then
             QT_CONFIG="$QT_CONFIG xcb-xlib"
         fi