Qt 5.15.0 cannot be compiled with options -xcb and -xcb-xlib on RedHat 7.5 with Gcc

Hello,

I need to compile Qt 5.15.0 on Linux with xcb, glx and opengl desktop support. 

My configure command is the following

configure -opensource -platform linux-g+-64 -shared -release -no-rpath -cups -system-zlib -cstd c+17 -opengl desktop -qt-libjpeg -qt-libpng -xcb -xcb-xlib -no-icu -nomake examples -nomake tests -prefix

I have all the packages on that machine as recommended on https://wiki.qt.io/Building_Qt_5_from_Git 

Yet, the configure fails reporting that the -xcb-xlib option is not usable because features.xib is not available with the error message.

ERROR: Feature 'xcb-xlib' was enabled, but the pre-condition 'features.xlib && libs.xcb_xlib' failed.

I have looked into the qtbase/src/gui/configure.json file and found

"xlib": {
 "label": "XLib",
 "test": {
 "main": [
 "Display *d = XOpenDisplay(NULL);",
 "XCloseDisplay(d);"
 ]
 },
 "headers": "X11/Xlib.h",
 "sources": [
 \{ "type": "makeSpec", "spec": "X11" }
 ]
 },

From what I figured out from that file, the X11 path is read from the mkspecs file for my configuration. However, in  qtbase/mkspecs/linux-g++-64/qmake.conf, the paths referenced are

QMAKE_LIBDIR_X11 = /usr/X11R6/lib64
QMAKE_LIBDIR_OPENGL = /usr/X11R6/lib64

and this is not at all the configuration on my RedHat machine.

I changed the paths to 

QMAKE_LIBDIR_X11 = /usr/lib64
QMAKE_LIBDIR_OPENGL = /usr/lib64

so that it matches my local configuration and my configuration command works now,

Would it be better to change the check in qtbase/src/gui/configure.json

to use a pkg-config pattern instead?

Something like 

"xlib": {
 "label": "XLib",
 "test": {
 "main": [
 "Display *d = XOpenDisplay(NULL);",
 "XCloseDisplay(d);"
 ]
 },
 "headers": "X11/Xlib.h",
 "sources": [
 *{ "type": "pkgConfig", "args": "x11" }*
 ]
 },

I have not tested it: I do not know if the Qt build system will handle possibly contradictory information for QMAKE_LIBDIR_X11 and QMAKE_LIBDIR_OPENGL: one coming from the qmake.defs and one from configure.json.

Best regards,

Didier Donner