Do not force Z_CONST for C++.

Forcing Z_CONST resulted in an issue when compiling Firefox.  Now
if someone wants to compile zlib as C++ code (which it isn't), now
they will need to #define Z_CONST themselves.

zconf.h

@@ -218,7 +218,7 @@
 #  endif
 #endif
 
-#if ( defined(ZLIB_CONST) || defined(__cplusplus) ) && !defined(z_const)
+#if defined(ZLIB_CONST) && !defined(z_const)
 #  define z_const const
 #else
 #  define z_const

zconf.h.cmakein

@@ -220,7 +220,7 @@
 #  endif
 #endif
 
-#if ( defined(ZLIB_CONST) || defined(__cplusplus) ) && !defined(z_const)
+#if defined(ZLIB_CONST) && !defined(z_const)
 #  define z_const const
 #else
 #  define z_const

zconf.h.in

@@ -218,7 +218,7 @@
 #  endif
 #endif
 
-#if ( defined(ZLIB_CONST) || defined(__cplusplus) ) && !defined(z_const)
+#if defined(ZLIB_CONST) && !defined(z_const)
 #  define z_const const
 #else
 #  define z_const