Fix libz and liblzma detection

If libz or liblzma are detected with pkg-config, AC_CHECK_HEADERS must
not be run because the correct CPPFLAGS aren't set. It is actually not
required have separate checks for LIBXML_ZLIB_ENABLED and HAVE_ZLIB_H.
Only check for LIBXML_ZLIB_ENABLED and remove HAVE_ZLIB_H macro.

Fixes bug 764657, bug 787041.
diff --git a/nanohttp.c b/nanohttp.c
index 9cd2292..2143718 100644
--- a/nanohttp.c
+++ b/nanohttp.c
@@ -63,7 +63,7 @@
 #ifdef HAVE_STRINGS_H
 #include <strings.h>
 #endif
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
 #include <zlib.h>
 #endif
 
@@ -145,7 +145,7 @@
     char *authHeader;	/* contents of {WWW,Proxy}-Authenticate header */
     char *encoding;	/* encoding extracted from the contentType */
     char *mimeType;	/* Mime-Type extracted from the contentType */
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     z_stream *strm;	/* Zlib stream object */
     int usesGzip;	/* "Content-Encoding: gzip" was detected */
 #endif
@@ -434,7 +434,7 @@
     if (ctxt->mimeType != NULL) xmlFree(ctxt->mimeType);
     if (ctxt->location != NULL) xmlFree(ctxt->location);
     if (ctxt->authHeader != NULL) xmlFree(ctxt->authHeader);
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     if (ctxt->strm != NULL) {
 	inflateEnd(ctxt->strm);
 	xmlFree(ctxt->strm);
@@ -817,7 +817,7 @@
 	if (ctxt->authHeader != NULL)
 	    xmlFree(ctxt->authHeader);
 	ctxt->authHeader = xmlMemStrdup(cur);
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     } else if ( !xmlStrncasecmp( BAD_CAST line, BAD_CAST"Content-Encoding:", 17) ) {
 	cur += 17;
 	while ((*cur == ' ') || (*cur == '\t')) cur++;
@@ -1273,7 +1273,7 @@
 int
 xmlNanoHTTPRead(void *ctx, void *dest, int len) {
     xmlNanoHTTPCtxtPtr ctxt = (xmlNanoHTTPCtxtPtr) ctx;
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     int bytes_read = 0;
     int orig_avail_in;
     int z_ret;
@@ -1283,7 +1283,7 @@
     if (dest == NULL) return(-1);
     if (len <= 0) return(0);
 
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     if (ctxt->usesGzip == 1) {
         if (ctxt->strm == NULL) return(0);
 
@@ -1424,7 +1424,7 @@
 	/* 1 for '?' */
 	blen += strlen(ctxt->query) + 1;
     blen += strlen(method) + strlen(ctxt->path) + 24;
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     /* reserve for possible 'Accept-Encoding: gzip' string */
     blen += 23;
 #endif
@@ -1468,7 +1468,7 @@
 		    ctxt->hostname, ctxt->port);
     }
 
-#ifdef HAVE_ZLIB_H
+#ifdef LIBXML_ZLIB_ENABLED
     p += snprintf(p, blen - (p - bp), "Accept-Encoding: gzip\r\n");
 #endif