qt5base: fix build issue with mesa3d w/out xcb

qt5base: fix build issue with mesa3d w/out xcb

The mesa's EGL/eglplatform.h header includes X11 headers unless the flag
MESA_EGL_NO_X11_HEADERS is defined[1].

A build issue happens when mesa3d is selected as then OpenGL EGL backend
but the XCB library is not selected. For instance, with this defconfig:
BR2_arm=y
BR2_cortex_a9=y
BR2_ARM_FPU_VFPV3D16=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_INIT_NONE=y
BR2_PACKAGE_MESA3D=y
BR2_PACKAGE_MESA3D_GALLIUM_DRIVER_ETNAVIV=y
BR2_PACKAGE_MESA3D_OPENGL_ES=y
BR2_PACKAGE_QT5=y
BR2_PACKAGE_QT5BASE_OPENGL_LIB=y
BR2_PACKAGE_QT5BASE_LINUXFB=y
BR2_PACKAGE_QT5BASE_EGLFS=y
BR2_PACKAGE_QT5WEBKIT=y

Also the odroid-mali EGL backend suffers the same problem.

This commit backports the patch from the Gerrit of Qt which is in
review[5]. It extends the QMAKE_CXXFLAGS with cflags contained in the
egl.pc file.

In this situation, the define MESA_EGL_NO_X11_HEADERS is given to the
compiler that prevent from including the missing X headers.

The issues QTBUG-61712[3] and QTBUG-66233[4] are opened in the Qt
tracker.

Fixes
http://autobuild.buildroot.net/results/8781a561ae1a89e4d70ddaba65d8817eabe3ce69

[1]: https://github.com/mesa3d/mesa/blob/79ee1b2ff0b85f4eeb4165d23a7943c28d3a3d93/include/EGL/eglplatform.h#L109-L125
[2]: https://codereview.qt-project.org/#/c/198906/
[3]: https://bugreports.qt.io/browse/QTBUG-61712
[4]: https://bugreports.qt.io/browse/QTBUG-66233

Cc: Julien CORJON <corjon.j@ecagroup.com>
Cc: Peter Seiderer <ps.report@gmx.net>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>