package/qt5/qt5base: fix building with correct optimization

Qt5 has predefined optimization flags depending if you're building for
size, for debug etc. These flags are defined in
mkspecs/common/gcc-base.conf:

QMAKE_CFLAGS_OPTIMIZE      = -O2
QMAKE_CFLAGS_OPTIMIZE_FULL = -O3
QMAKE_CFLAGS_OPTIMIZE_DEBUG = -Og
QMAKE_CFLAGS_OPTIMIZE_SIZE = -Os

Then, in the same file, they use them to set
QMAKE_CFLAGS_RELEASE/QMAKE_CXXFLAGS_RELEASE:

QMAKE_CFLAGS_RELEASE       += $$QMAKE_CFLAGS_OPTIMIZE
QMAKE_CXXFLAGS_RELEASE    += $$QMAKE_CFLAGS_RELEASE

At this point there is our chance to override QMAKE_CFLAGS_OPTIMIZE_* in
qmake.conf, but it's too late, because QMAKE_CFLAGS_RELEASE is already
set (i.e. -O2) so trying to add or remove QMAKE_CFLAGS_OPTIMIZE (that is
reset now on) from QMAKE_CLAGS_RELEASE in
common/features/default_post.prf won't work:

optimize_size {
    !isEmpty(QMAKE_CFLAGS_OPTIMIZE):!isEmpty(QMAKE_CFLAGS_OPTIMIZE_SIZE)  {
        QMAKE_CFLAGS_RELEASE -= $$QMAKE_CFLAGS_OPTIMIZE
        QMAKE_CXXFLAGS_RELEASE -= $$QMAKE_CFLAGS_OPTIMIZE
        QMAKE_CFLAGS_RELEASE += $$QMAKE_CFLAGS_OPTIMIZE_SIZE
        QMAKE_CXXFLAGS_RELEASE += $$QMAKE_CFLAGS_OPTIMIZE_SIZE
    }
} else: optimize_full {
    !isEmpty(QMAKE_CFLAGS_OPTIMIZE):!isEmpty(QMAKE_CFLAGS_OPTIMIZE_FULL)  {
        QMAKE_CFLAGS_RELEASE -= $$QMAKE_CFLAGS_OPTIMIZE
        QMAKE_CXXFLAGS_RELEASE -= $$QMAKE_CFLAGS_OPTIMIZE
        QMAKE_CFLAGS_RELEASE += $$QMAKE_CFLAGS_OPTIMIZE_FULL
        QMAKE_CXXFLAGS_RELEASE += $$QMAKE_CFLAGS_OPTIMIZE_FULL
    }
}

So let's reset:
QMAKE_CFLAGS_RELEASE
QMAKE_CFLAGS_DEBUG
QMAKE_CXXFLAGS_RELEASE
QMAKE_CXXFLAGS_DEBUG
in our qmake.conf since the only assignment done in
mkspecs/common/gcc-base.conf only regards optimization.

This package is also affected by BR2_TOOLCHAIN_HAS_GCC_BUG_90620 and
it's been worked around by appending -O0 to CFLAGS/CXXFLAGS. This bug
prevented workaround to work overriding optimization flags, so solving
this also solves workaround problem.

Fixes:
http://autobuild.buildroot.net/results/ada/adaa9b4bcc6f9d2b5e82c479859a07e8abf5cf13/
http://autobuild.buildroot.net/results/a83/a83bdd1f3bf309c07abebe871b017c331ed36e67/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Tested-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Arnout: add a comment to qmake.conf.in]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit c4a6f974b158a23bcab2f5e0e42971d0c15cc04b)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/qt5/qt5base/qmake.conf.in

@@ -15,10 +15,15 @@ QMAKE_CXX               = $${CROSS_COMPILE}g++
 # modifications to gcc-base.conf
 QMAKE_CFLAGS           += $${BR_COMPILER_CFLAGS}
 QMAKE_CXXFLAGS         += $${BR_COMPILER_CXXFLAGS}
+# Remove all optimisation flags, we really only want our own.
 QMAKE_CFLAGS_OPTIMIZE       =
 QMAKE_CFLAGS_OPTIMIZE_DEBUG =
 QMAKE_CFLAGS_OPTIMIZE_FULL  =
 QMAKE_CFLAGS_OPTIMIZE_SIZE  =
+QMAKE_CFLAGS_DEBUG =
+QMAKE_CXXFLAGS_DEBUG =
+QMAKE_CFLAGS_RELEASE =
+QMAKE_CXXFLAGS_RELEASE =
 CONFIG                 += nostrip
 
 QMAKE_LIBS             += -lrt -lpthread -ldl