Improved autoconf script

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