Unconditionally including ciso646 in qglobal.h to check for libc++ makes
including qglobal.h from C programs fail.
qglobal.h is not supposed to be used in C programs, but kdelibs4's
FindQt.cmake ends up calling CHECK_SYMBOL_EXISTS() instead of
CHECK_CXX_SYMBOL_EXISTS() to check if some macros are defined. Since
kdelibs4 is in maintenance mode, our changes to it can be avoided if we just
check for __cplusplus before including ciso646 (<initializer_list> is not
available in C anyway).
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