Reduce lack of -x to just an information messag

[PaulWessel]

See #8300. Just say if no parallel support then only once care will be used. However @seisman, my effort at printing that out fails (but the error is gone). Can you check why

#if !defined(GMT_MP_ENABLED)

seems to fail for me? E.g., lines 9169 and 18779 in gmt_init.c

[seisman]

I'm getting this error when OpenMP is disabled:

/usr/bin/ld: src/libgmt.so.6.6.0: undefined reference to `gmtinit_parse_x_option'

The reason is gmtinit_parse_x_option is only defined if OpenMP is enabled.