configure.ac: configure.ac: add PKG_CHECK_MODULES and AC_CHECK_LIB/ACHECK_HEADERS for librtpi

[alyxazon]

Implement librtpi checks in configure.ac (based on Enkhbold's idea, #160)

What happens if...

No librtpi is installed:

Configure:

checking for etherbone >= 2.1.0... yes
checking for sigc++-2.0... yes
checking for librtpi >= 1.0.1... no
checking for pi_mutex_alloc in -lrtpi... no
checking for rtpi.h... no

Make:

saftbus/client.hpp:35:10: fatal error: rtpi/mutex.hpp: No such file or directory
   35 | #include <rtpi/mutex.hpp>
      |          ^~~~~~~~~~~~~~~~
compilation terminated.

Official librtpi 1.0.1 is installed

Configure:

checking for etherbone >= 2.1.0... yes
checking for sigc++-2.0... yes
checking for librtpi >= 1.0.1... no
checking for pi_mutex_alloc in -lrtpi... yes
checking for rtpi.h... yes

Make:
Success!

GSI-Fork (librtpi) or fake bel_projects 1.0.2 dot pc file:

Configure:

checking for etherbone >= 2.1.0... yes
checking for sigc++-2.0... yes
checking for librtpi >= 1.0.1... yes

Make:
Success!