The list is now shorter, only include very common compilers, and is consistent
in C and C++. In particular, on OSX it chooses both clang++ and clang as default.

Fixes #375

Allows embedding a separate PLUMED instance.

Using auto when creating a unique_ptr

Using c++11 to initialize the map instead of calling a function everytime

This is to avoid potential clashes.

I removed many cases that are not in practice useful. In particular,
it won't be anymore possible to:
- Link statically PLUMED if both "ar" and "ld -ro" do not work correctly.
- Link runtime PLUMED is "ar" does not work correctly.

Both statements are true only for installed PLUMED. Not installed plumed
still uses object files.

(see #352)

All atoms were erroneously requested also after the first step.

Allow COMMITTOR to stop driver.

when using DEBUG, detailed timers were always shown by mistake.
Now they are only shown when using flag DETAILED_TIMERS

I added a configure flag --disable-absolute-install-name
to allow OSX shared libraries to behave such as Linux libraries

This will need DYLD_LIBRARY_PATH to be set, which is not a good idea.

I have to add some documentation.

Now that libraries are separated (libplumed and libplumedKernel)
it is necessary that both are linked with the --coverage flag
or coverage build will fail.

On OSX, it is standard to hardcode the absolute path of a library in its install_name.
On Linux, the standard behavior so to only hardcode the soname and let
the system find the library. This actually requires setting LD_LIBRARY_PATH.

I added the possibility to use ./configure --enable-absolute-soname
to make Linux behave such as OSX.

I took the occasion to simplify considerably the linking of shared libraries.
Now the same command is used on Linux and OSX.

- do not set LD_LIBRARY_PATH on OSX
- set LIBRARY_PATH also when dynamic libraries are disabled

Allow avoiding rpath-link. In addition, it will make
absolute sonames on linux possible (later commit).