Bug introduced on 6f8df85d

Warning at the end of configure about missing MPI was incorrect, as a consequence.
of 971a5078. Fixed now

[ci skip]

[makedoc]

Add more regtest for DRR module

It doesn't run on travis-ci, but on my mac it runs successfully.

[makedoc]

Can be used to skip a regtest based on some complex test.
E.g., to skip a test on mac add this to the config file:

function plumed_custom_skip(){
  if [ "$(uname)" == "Darwin" ] ; then
    return 0
  fi
}

Small typo in the regtests log

boost::serialization doesn't offer forward compatibility. This means
we cannot deserialize an archive from newer boost on old boost.

[makedoc]

[makedoc]

1. Add test for UI estimator
2. Add test for merging windows in drr_tool
3. Add restart test

[makedoc]

This allows port "plumed +allmodules" to use DRR module.

[makedoc]

Macros that look for system libraries have been simplified in many respects:
1. Search for libs is now made with AC_SEARCH_LIBS, which does the same
   thing our macro was doing (try first with no explicitly linked lib, then try with
   libs) but is standard.
2. Actually, AC_SEARCH_LIBS is wrapped by PLUMED_SEARCH_LIBS that also
   follows --disable-libsearch correctly.
3. PLUMED_CHECK_CXX_PACKAGE has been added which simplifies searching
   c++ libs by providing a short test program (closes #274).
4. Both PLUMED_CHECK_PACKAGE and PLUMED_CHECK_CXX_PACKAGE now also
   set an env var that can be used later in the configure script
5. Both PLUMED_CHECK_PACKAGE and PLUMED_CHECK_CXX_PACKAGE accept multiple
   candidate libraries (similarly to AC_SEARCH_LIBS)
6. Blas, lapack, gsl, and molfile are now searched with PLUMED_SEARCH_LIBS

Got rid of first argument. Now, the name of the variable is computed
automatically from the option by replacing - with _

No change in ./configure file

[makedoc]

[makedoc]

Add extended adaptive biasing force or dynamic reference restraining method