mongoose: fix static build issue

Fixes:
http://autobuild.buildroot.net/results/372/372515ba0a09a23237ae34024658f21c2625d6e7/



Disable the shared library support in mongoose, as the only thing it does is
including dlfcn.h - So it is useless.  Also remove the unneeded -ldl from
the web server linker flags.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>