Merge branch 'bugfix/julien/install-dir' into 'master'

[Makefile] fix 'make install' when E-ACSL is compiled/installed within Frama-C

'make install' used ```FRAMAC_SHARE``` while it should have used ```FRAMAC_DATADIR```. The former does not contain the right value when compiled inside Frama-C.

See merge request !47