Instead of using TARGET_CFLAGS and EXTRA_CFLAGS in cmake and scons
build use the TARGET_CXXFLAGS and EXTRA_CXXFLAGS like it is done for
normal make and configure. configure used TARGET_CXXFLAGS and
EXTRA_CFLAGS for the CXXFLAGS. The package-default.mk sets
"EXTRA_CXXFLAGS = $(EXTRA_CFLAGS)" so using EXTRA_CXXFLAGS flags should
be save.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>