Merge branch 'feature/patrick/framac-installdir' into 'master'

[install] allows to use FRAMAC_INSTALLDIR when PREFIX is undefined

See merge request frama-c/frama-c!4040

share/Makefile.common

@@ -54,12 +54,16 @@ PLATFORM:=$(shell uname -s)
 #############
 
 ifndef PREFIX
+ifdef FRAMAC_INSTALLDIR
+PREFIX=$(FRAMAC_INSTALLDIR)
+else
 ifdef OPAM_SWITCH_PREFIX
 PREFIX=$(OPAM_SWITCH_PREFIX)
 else
 PREFIX=/usr/local
 endif
 endif
+endif
 
 #############
 # Verbosing #