configure.ac: Fix llhttplib substituion.

* configure.ac (@http_lib@): Rename to llhttp_lib, replace the substituted value for a pkg-config module name.

configure.ac

@@ -184,7 +184,7 @@ AM_COND_IF([PROXY_CLIENT_OR_SERVER], [
     # llhttp has no pkgconfig, instead we check with:
     AC_CHECK_LIB(llhttp, exit,, AC_MSG_ERROR([Missing llhttp library files]))
     AC_CHECK_HEADERS([llhttp.h], [llhttp_headers=yes; break;])
-    AC_SUBST(http_lib, ["-lllhttp"])
+    AC_SUBST(llhttp_lib, [", liblllhttp"])
     AS_IF([test "x$llhttp_headers" != "xyes"], AC_MSG_ERROR([Missing llhttp headers files]))
 ])