Prevent invalid inclusions when HAVE_* is set to 0.

When including zconf.h from an external project that already has HAVE_UNISTD_H declared to a non-empty define (in this case =0), the inclusion of unistd.h is incorrectly applied. This breaks inclusion of zlib in several projects on windows and other platforms without unistd.h. This patch changes behaviour so that HAVE_UNISTD_H is checked to see if it is defined as blank or not equal to '0' before inclusion of the header.

zconf.h.cmakein

@@ -434,11 +434,19 @@ typedef uLong FAR uLongf;
 #endif
 
 #ifdef HAVE_UNISTD_H    /* may be set to #if 1 by ./configure */
-#  define Z_HAVE_UNISTD_H
+#  if ~(~HAVE_UNISTD_H + 0) == 0 && ~(~HAVE_UNISTD_H + 1) == 1
+#    define Z_HAVE_UNISTD_H
+#  elif HAVE_UNISTD_H != 0
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 
 #ifdef HAVE_STDARG_H    /* may be set to #if 1 by ./configure */
-#  define Z_HAVE_STDARG_H
+#  if ~(~HAVE_STDARG_H + 0) == 0 && ~(~HAVE_STDARG_H + 1) == 1
+#    define Z_HAVE_STDARG_H
+#  elif HAVE_STDARG_H != 0
+#    define Z_HAVE_STDARG_H
+#  endif
 #endif
 
 #ifdef STDC

zconf.h.in

@@ -432,11 +432,19 @@ typedef uLong FAR uLongf;
 #endif
 
 #ifdef HAVE_UNISTD_H    /* may be set to #if 1 by ./configure */
-#  define Z_HAVE_UNISTD_H
+#  if ~(~HAVE_UNISTD_H + 0) == 0 && ~(~HAVE_UNISTD_H + 1) == 1
+#    define Z_HAVE_UNISTD_H
+#  elif HAVE_UNISTD_H != 0
+#    define Z_HAVE_UNISTD_H
+#  endif
 #endif
 
 #ifdef HAVE_STDARG_H    /* may be set to #if 1 by ./configure */
-#  define Z_HAVE_STDARG_H
+#  if ~(~HAVE_STDARG_H + 0) == 0 && ~(~HAVE_STDARG_H + 1) == 1
+#    define Z_HAVE_STDARG_H
+#  elif HAVE_STDARG_H != 0
+#    define Z_HAVE_STDARG_H
+#  endif
 #endif
 
 #ifdef STDC