steal more (good) stuff from mandoc-portable :)

While here, move the SRCS variable to the configure and add the -includes in Makefile.local; it de-clutters the Makefile a bit.

Add to gmid the ability to forwad a request to another gemini server and thus acting like a reverse proxy. The current syntax for the config file is server "example.com" { ... proxy relay-to host:port } Further options (like the use of custom certificates) are planned. cf. github issue #7