Removed a few more unused configuration macros.

I've left HAVE_LIBPTHREAD as a CMake variable, since TBB's build depends on it.

Some macros in internal.hpp depended on HAVE_ALLOCA_H, but they were, in turn,
unused, so I've just deleted them all.

modules/core/src/persistence.cpp

@@ -59,7 +59,6 @@
 #endif
 
 #if USE_ZLIB
-#  undef HAVE_UNISTD_H //to avoid redefinition
 #  ifndef _LFS64_LARGEFILE
 #    define _LFS64_LARGEFILE 0
 #  endif