Fixed issue with "make install"

Before this fix, attempting to "make install" without "make" first
could result in corrupted path in the Plumed.inc/Plumed.cmake
files (the one used to patch MD codes). This resulted in linking
errors when trying to use "plumed patch --shared"
Merge branch 'v2.0' into v2.1
changelog
Added error message that was missing in grids

(cherry picked from commit ab739533b70f35129e3b0708f9184cd03833e7da)
Merge branch 'v2.0' into v2.1
Changelog fixed

In commit 606b65a4ab716fc4cef46a5ffd493509c4d09ec2
I wrote that error was due to user typing "make install"
without "make". I now realized that the error was always there
when PLUMED was installed. I modify the CHANGELOG to make
it more clear. I will also send a patch on the mailing list.
Merge branch 'v2.0' into v2.1
Installation: it is better to compile ALMOST with -fPIC
Merge branch 'v2.1' into v2.1-openmp