Added #ifndef around NGX_HAVE_CASELESS_FILESYSTEM define.

This brings Cygwin compilation in line with other case-insensitive
systems (notably win32 and OS X) where one can force case sensitivity
using -DNGX_HAVE_CASELESS_FILESYSTEM=0.

src/os/unix/ngx_files.h

diff --git a/src/os/unix/ngx_files.h b/src/os/unix/ngx_files.h
index 4b20165..a78ec96 100644
--- a/src/os/unix/ngx_files.h
+++ b/src/os/unix/ngx_files.h
@@ -53,7 +53,9 @@
 
 #ifdef __CYGWIN__
 
+#ifndef NGX_HAVE_CASELESS_FILESYSTEM
 #define NGX_HAVE_CASELESS_FILESYSTEM  1
+#endif
 
 #define ngx_open_file(name, mode, create, access)                            \
     open((const char *) name, mode|create|O_BINARY, access)