Clean up CMake header and type checks.

Unused macros: HAVE_DLFCN_H, HAVE_INTTYPES_H, HAVE_MEMORY_H,
HAVE_STDLIB_H, HAVE_STRINGS_H, HAVE_STRING_H, HAVE_SYS_BYTESWAP_H,
HAVE_SYS_STAT_H, HAVE_SYS_TYPES_H, HAVE_UNISTD_H.

Used but never set macros: HAVE_LIBLZF, HAVE_LIBQUICKLZ. These only gate
conditional includes. The code that takes advantage of them was removed.

Unused types: ssize_t.

The testing code uses HAVE_FUNC_MMAP, which was not wired in the CMake
build, causing a whole test to be skipped.

snappy-test.h

@@ -110,16 +110,6 @@
 #include "lzo/lzo1x.h"
 #endif
 
-#ifdef HAVE_LIBLZF
-extern "C" {
-#include "lzf.h"
-}
-#endif
-
-#ifdef HAVE_LIBQUICKLZ
-#include "quicklz.h"
-#endif
-
 namespace {
 
 namespace file {