Compile with -Wsuggest-attribute=malloc.

Compile with -Wsuggest-attribute=malloc.

We already employ this attribute extensively to help the compiler
improve optimization. However, a few malloc-like functions were not
yet marked with __malloc. Fix that and enable the warning to make
sure that new malloc-like functions get marked.

Since not all supported compilers know about this warning option, we
need to check at compile time whether the option is supported. Thanks
to the existing cc-option make(1) function, this is a simple one-liner
for Makefile.real.