Move the requirement checks within the respective plugin cmakefile.
Use REQUIRED instead of the manual FOUND check and error handling.
Note: the tcl check was only moved, since using REQUIRED explodes in
CI.
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Allows for custom LIBDIR to specified with cmake builds. WEECHAT_LIBDIR
is used for a custom Weechat directory, or set from LIBDIR (as LIBDIR
used to be set).