libbsd-overlay needs to be declared properly with the correct AM_CFLAGS, but this only happens via a common Makefile.common.in template which gets filled out at ./configure time. This properly ensures libbsd compat libraries, such as getprogname() are correctly found on the system.

Move template into its own subproject as it's only needed at compile-time and shouldn't be installed. This needs compiling with the host compiler which is configurable via HOSTCC and HOSTCLFAG environment variables. This diff largely from op@ but additional tweaking from myself.

it's only needed at build time to generate a source file.

was added there mistakenly. it's not part of the "compiler", it's part of the "user library" and provides useful functions for the consumers. see also 1dec34dfda7ccb3caadab65aca907251ab8fd72e

This updates -portable's gotwebd instance to make use of the new templating engine.