These modules should not be compulsory

plumed core can be compiled without these.

With this change it is necessary to solve a but in the script building the
list of modules to be tested. I took the occasion to simplify it.
Now, all the optional modules are checked, but if a directory is
not present in regtest/ that module is ignored.
Small improvements in configure script

Added possibility to remove checks such as
"-ansi" and "-Wall" (--disable-basic-warnings)

Changed name of temporary files from test... to conftest...
(consistent with autoconf behavior)