Dynamic modules.

The auto/module script is extended to understand ngx_module_link=DYNAMIC.
When set, it links the module as a shared object rather than statically
into nginx binary.  The module can later be loaded using the "load_module"
directive.

New auto/module parameter ngx_module_order allows to define module loading
order in complex cases.  By default the order is set based on ngx_module_type.

3rd party modules can be compiled dynamically using the --add-dynamic-module
configure option, which will preset ngx_module_link to "DYNAMIC" before
calling the module config script.

Win32 support is rudimentary, and only works when using MinGW gcc (which
is able to handle exports/imports automatically).

In collaboration with Ruslan Ermilov.
diff --git a/auto/lib/conf b/auto/lib/conf
index e1e4475..6aaa43a 100644
--- a/auto/lib/conf
+++ b/auto/lib/conf
@@ -58,11 +58,11 @@
     . auto/lib/zlib/conf
 fi
 
-if [ $USE_LIBXSLT = YES ]; then
+if [ $USE_LIBXSLT != NO ]; then
     . auto/lib/libxslt/conf
 fi
 
-if [ $USE_LIBGD = YES ]; then
+if [ $USE_LIBGD != NO ]; then
     . auto/lib/libgd/conf
 fi
 
@@ -70,7 +70,7 @@
     . auto/lib/perl/conf
 fi
 
-if [ $HTTP_GEOIP = YES ]; then
+if [ $USE_GEOIP != NO ]; then
     . auto/lib/geoip/conf
 fi