Include QMAKE_CC when extracting C compiler flags

This fixes cross-compile settings detection for cross-compilation
environments where the C compiler cross-compilation flags are set in
CC/QMAKE_CC instead of CFLAGS/QMAKE_CFLAGS (e.g. OpenEmbedded).

Change-Id: I0a7b0f6c47a00775ec18073a16359c8351363285
Reviewed-by: Andras Becsi <andras.becsi@digia.com>