In 3.0.4, the horrible makefiles override the CXXFLAGS variable:

firebird/builds/posix/prefix.linux_generic:CXXFLAGS=-std=gnu++03

Work around that by passing LO's customised CXXFLAGS as a command-line
parameter to make, which in turn overrides the definition in the
makefile.

Other platforms, in particular Darwin, apparently extend CXXFLAGS
instead of overwriting.

To nobody's surprise, gnu++03 doesn't even build with clang because ICU
headers use C++11 features (although strangely gcc doesn't complain).

Change-Id: If3b26482a4f4bf284057e261677cd7182656154f
Reviewed-on: https://gerrit.libreoffice.org/67255
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>