Configure/build changes and fixes:

*) Revamped "configure --help" text.

*) FreeBSD 10-current has recently gotten POSIX_FADV_* macros.
   A fix for the broken build applied.

*) AIX 7 defines sys_nerr in errno.h, therefore <errno.h> included
   in the sys_nerr test.

   When sys_nerr and _sys_nerr are missed on a particular platform
   and our euristic for a maximum errno detection applied always
   print the maximum errno number we reached instead of printing void.

   This makes possible to build nginx on AIX 7.1.

*) Made it possible to build/install from the SVN checkout.

r4106, r4107, r4108, r4113, r4156, r4157:

*) ngx_http_mp4_module.
*) "video/mp4" mime type added.