to match the visibility of max_align_t between C and C++.
GCC 4.9 defaults to C89 and does not accept the "restrict" keyword
by default. This causes a configure check to add -std=gnu99 to the
compiler flags. Since max_align_t is from C11, the -std=gnu99 flag
removes it from visibility and gnutls's copy of gnulib then provides
its own replacement definition of max_align_t. When gnutls builds
its C++ library, the C++ visibility rules are different. The
max_align_t from stddef.h reappears and collides with the gnulib
replacement.