BUG/MINOR: jit: don't rely on USE flag to detect support

Since ea68d36 we show whether JIT is enabled, based on the USE-flag
(USE_PCRE_JIT). This is too naive; libpcre may be built without JIT
support (which is the default).

Fix this by calling pcre_config(), which has the accurate information
we are looking for.

Example of a libpcre without JIT support after this patch:
> ./haproxy -vv | grep PCRE
>   OPTIONS = USE_STATIC_PCRE=1 USE_PCRE_JIT=1
> Built with PCRE version : 8.32 2012-11-30
> PCRE library supports JIT : no (libpcre build without JIT?)
diff --git a/src/haproxy.c b/src/haproxy.c
index d4c1189..da7a625 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -292,13 +292,20 @@
 
 #ifdef USE_PCRE
 	printf("Built with PCRE version : %s", pcre_version());
-	printf("\nPCRE library supports JIT : "
-#ifndef USE_PCRE_JIT
-	       "no (USE_PCRE_JIT not set)"
+	printf("\nPCRE library supports JIT : ");
+#ifdef USE_PCRE_JIT
+	{
+		int r;
+		pcre_config(PCRE_CONFIG_JIT, &r);
+		if (r)
+			printf("yes");
+		else
+			printf("no (libpcre build without JIT?)");
+	}
 #else
-	       "yes"
+	printf("no (USE_PCRE_JIT not set)");
 #endif
-	       "\n");
+	printf("\n");
 #else
 	printf("Built without PCRE support (using libc's regex instead)\n");
 #endif