cmnghttp2: check for HAVE_SIZEOF_SSIZE_T and not HAVE_SSIZE_T

The `check_type_size(ssize_t SIZEOF_SSIZE_T` call in cmcurl (referenced
by the comment above) defines `HAVE_SIZEOF_SSIZE_T` and not
`HAVE_SSIZE_T`.  The `HAVE_SSIZE_T` variable *might* get defined, but
via the `CHECK_TYPE_SIZE(ssize_t SSIZE_T)` call in cmlibarchive, which
would be configured *after* cmnghttp2, and so the first configure would
lead to an invalid `cmnghttp2/config.h` file.

Utilities/cmnghttp2/CMakeLists.txt

@@ -7,10 +7,10 @@ elseif(CMAKE_C_COMPILER_ID STREQUAL "PathScale")
 endif()
 
 # Re-use some check result cache entries from cmcurl:
-# * HAVE_ARPA_INET_H
-# * HAVE_NETINET_IN_H
-# * HAVE_SSIZE_T
-if(NOT HAVE_SSIZE_T)
+# * HAVE_ARPA_INET_H    (referenced in cmakeconfig.h.in)
+# * HAVE_NETINET_IN_H   (referenced in cmakeconfig.h.in)
+# * HAVE_SIZEOF_SSIZE_T (referenced here)
+if(NOT HAVE_SIZEOF_SSIZE_T)
   set(ssize_t KWIML_INT_intptr_t)
 endif()
 configure_file(cmakeconfig.h.in config.h)