Port C11 and C++11 testing to clang

Port C11 and C++11 testing to clang

* lib/autoconf/c.m4 (_AC_C_C99_TEST_HEADER):
Include stddef.h, for offsetof.
(_AC_PROG_CC_C11): Limit _Static_assert to integer constant
expressions.  Suggested by Nick Bowler in:
http://lists.gnu.org/archive/html/autoconf/2016-02/msg00009.html
(_AC_CXX_CXX11_TEST_BODY): Don't use string literals to initialize
non-const pointers.  Suggested by Mike Miller in:
http://lists.gnu.org/archive/html/autoconf/2016-02/msg00008.html