package/qt5/qt5base: fix build with ccache

Building with ccache failed with:

    Running configuration tests...
    Failed to process makespec for platform 'devices/linux-buildroot-g++'
    Project ERROR: Compiler <path_to_output_dir>/host/usr/bin/ccache <path_to_output_dir>/host/usr/bin/<cross_compile>-g++ not found. Check the value of CROSS_COMPILE -device-option
    Could not read qmake configuration file <path_to_output_dir>/build/qt5base-5.5.0/mkspecs/devices/linux-buildroot-g++/qmake.conf.
    Error processing project file: /dev/null

This was caused by Buildroot setting this in
qt5base-5.5.0/mkspecs/devices/linux-buildroot-g++/qmake.conf:

    QMAKE_CXX               = $${BR_CCACHE} $${CROSS_COMPILE}g++

But qt5base-5.5.0/mkspecs/features/device_config.prf expects QMAKE_CXX
to be a single valid (absolute or QMAKE_PATH_ENV-relative) path to an
existing file, which is not possible if using ccache as above.

Add a patch fixing this by testing only the first value in QMAKE_CXX.

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>