Merge of r4961: configure: better check for PCRE JIT.

On Mac OS X system toolchain by default prefers include files
from /usr/local/include, but libraries from /usr/lib.  This might result in
various problems, in particular the one outlined below.

If the PCRE library is installed into /usr/local/, this results in pcre.h
being used from /usr/local/include (with PCRE_CONFIG_JIT defined), but
libpcre from /usr/lib (as shipped with the OS, without pcre_free_study()
symbol).  As a result build fails as we use pcre_free_study() function
if we try to compile with PCRE JIT support.

Obvious workaround to the root cause is to ask compiler to prefer
library from /usr/local/lib via ./configure --with-ld-opt="-L/usr/local/lib".
On the other hand, in any case it would be good to check if the function
we are going to use is available, hence the change.

See thread here for details:
http://mailman.nginx.org/pipermail/nginx-devel/2012-December/003074.html

Prodded by Piotr Sikora.

auto/lib/pcre/conf

@@ -172,6 +172,7 @@ else
             ngx_feature="PCRE JIT support"
             ngx_feature_name="NGX_HAVE_PCRE_JIT"
             ngx_feature_test="int jit = 0;
+                              pcre_free_study(NULL);
                               pcre_config(PCRE_CONFIG_JIT, &jit);
                               if (jit != 1) return 1;"
             . auto/feature