Make copies of cstdlib/cmath and use them [BZ #20314]
If C++ headers <cstdlib> or <cmath> are used, GCC 6 will include
/usr/include/stdlib.h or /usr/include/math.h from "#include_next"
(instead of stdlib/stdlib.h or math/math.h in the glibc source
directory), and this turns up as a make dependency. An implicit
rule will kick in and make will try to install stdlib/stdlib.h or
math/math.h as /usr/include/stdlib.h or /usr/include/math.h because
the target is out of date. We make a copy of <cstdlib> and <cmath>
in the glibc build directory so that stdlib/stdlib.h and math/math.h
will be used instead of /usr/include/stdlib.h and /usr/include/math.h.
[BZ #20314]
* Makeconfig (CXXFLAGS): Prepend -I$(common-objpfx).
* Makerules (before-compile): Add $(common-objpfx)cstdlib and
$(common-objpfx)cmath.
($(common-objpfx)cstdlib): New target.
($(common-objpfx)cmath): Likewise.