Allow dynamic linking of mbedtls

[thyssentishman]

I'd like to package rirc for OpenBSD for which having mbedtls dynamically linked (or included in a release) would be necessary. The porting infrastructure of OpenBSD does not allow network access to the build process and therefore mbedtls can not be fetched.

[rcr]

I support this being optional. Is there an easy/standard mechanism from the bsd packaging perspective to inform Make to either statically or dynamically link?