Add and use has() and path_of() funcs

Add has() and path_of() funcs and use them across configure; has()
will test whether a command or builtin is available; path_of() will
search the PATH for executables and return the full pathname if found.

Signed-off-by: Loïc Minier <lool@dooz.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>