Part of a wider change.

On some systems libmd isn't explicitly implied, so detect this via pkg-config at configure time. Problem reported by @stsp

Rather than assume all dependencies are required for all programs, split them out. This will make packaging easier, as well as splitting the code to use subprojects. Note that due to the use of config.h semantics, in most cases the got_compat.h header file is now at the top of the .c file it is included in, so that it can handle the system header inclusion properly.

sha1.c -> hash.c

Provide the ability for -portable to continue to compile, until such time that gotd and gotsh have been enabled in -portable.

Allow the libexec helper, `got-read-patch` to be compiled, portably.