This was introduced for conditional compilation of the code that requires
the zlib library.

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 is 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.

Now perl configure will correctly fail if ExtUtils::Embed perl module
is not present in the system (found on Amazon Linux AMI, as of
release 2012.03).

Parentheses are more portable, in particular they are understood by nmake
while braces aren't.

It is now needed for gunzip filter.

Steps to reproduce:

./configure --with-cc="gcc46" --with-cc-opt="-Wall -Werror -O2"