libstdc++: Add system_header pragma to <bits/c++config.h>

Without this change many tests that depend on an effective-target will
fail when compiled with -pedantic -std=c++98. This happens because the
preprocessor check done by v3_check_preprocessor_condition uses -Werror
and includes <bits/c++config.h> directly (rather than via another header
like <string>). If <bits/c++config.h> is not a system header then this
pedwarn is not suppressed, and the effective-target check fails:

bits/c++config.h:220: error: anonymous variadic macros were introduced in C++11 [-Werror=variadic-macros]
cc1plus: all warnings being treated as errors
compiler exited with status 1
UNSUPPORTED: 18_support/headers/limits/synopsis.cc

We could consider also changing proc v3_check_preprocessor_condition so
that it includes a real header, rather than just <bits/c++config.h>, but
that's not necessary for now.

libstdc++-v3/ChangeLog:

	* include/bits/c++config: Add system_header pragma.

0 files changed, 0 insertions, 0 deletions