--enable-rpath also use to PREFIX/lib

[EricDeveaud]

Description

when configure --enable-rpath --prefix=/non/standard/location build does not set RPATH to PREFIX/lib
that brings the following error while installed

rpm_maker:plumed/plumed-2.7.2 > plumed -h 
plumed: error while loading shared libraries: libplumedKernel.so: cannot open shared object file: No such file or directory

the follwoing diff add PREFIX/lib to the rpath

Target release

plumed-2.7.2

regards
Eric

[codecov-commenter]

Codecov Report

Merging #767 (a722835) into master (41ceef4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #767   +/-   ##
=======================================
  Coverage   85.77%   85.77%           
=======================================
  Files         594      594           
  Lines       48335    48335           
=======================================
  Hits        41460    41460           
  Misses       6875     6875           

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[GiovanniBussi]

Thanks! Notice that the right path is $libdir.

I fixed this in master and not in 2.7 since I am not sure it is really a bug fix, rather a preference. If you are aware of some other code following this convention I am happy to backport it.