Add a ./config --solo option to make zlib subset with no libary use

A common request has been the ability to compile zlib to require no
other libraries.  This --solo option provides that ability.  The price
is that the gz*, compress*, and uncompress functions are eliminated,
and that the user must provide memory allocation and free routines to
deflate and inflate when initializing.

zutil.c

@@ -85,7 +85,11 @@ uLong ZEXPORT zlibCompileFlags()
 #ifdef FASTEST
     flags += 1L << 21;
 #endif
+#ifdef Z_SOLO
+    return flags;
+#else
     return flags + gzflags();
+#endif
 }
 
 #ifdef DEBUG
@@ -157,6 +161,7 @@ void ZLIB_INTERNAL zmemzero(dest, len)
 }
 #endif
 
+#ifndef Z_SOLO
 
 #ifdef SYS16BIT
 
@@ -292,3 +297,5 @@ void ZLIB_INTERNAL zcfree (opaque, ptr)
 }
 
 #endif /* MY_ZCALLOC */
+
+#endif /* !Z_SOLO */