Fix memory allocation when GCRYCTL_ENABLE_M_GUARD.
* configure.ac: Add check for ALIGNOF_LONG_DOUBLE.
* src/stdmem.c: Adjust EXTRA_ALIGN.
Not sure if this get malloc alignment correct on all architectures. For example, align of long double on i386 is 4, but glibc documentation says that "The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight (or sixteen on 64-bit systems)" (https://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html).
Thank you for your comment.
I tried to improve the use_m_guard feature... but Werner suggested it is better to remove it, as we now have better tooling, like valgrind.