[makefile] use appropriate prefix to refer to source files

current dir might not be E-ACSL's main dir. For instance when the
plug-in is included in the main distribution 

src/plugins/e-acsl/Makefile.in

@@ -82,13 +82,15 @@ PLUGIN_CMO:= local_config \
 PLUGIN_HAS_MLI:=yes
 PLUGIN_DISTRIBUTED:=yes
 
-# Suppress a spurious warning in ocaml 4.04.0
-mmodel_analysis.cmo mmodel_analysis.cmi: E_ACSL_BFLAGS+= -w -60
-mmodel_analysis.cmx: E_ACSL_OFLAGS+= -w -60
 
 # We "save" this variable so that it can be used once PLUGIN_DIR has been reset
 EACSL_PLUGIN_DIR:=$(PLUGIN_DIR)
 
+# Suppress a spurious warning in ocaml 4.04.0
+$(EACSL_PLUGIN_DIR)/mmodel_analysis.cmo \
+$(EACSL_PLUGIN_DIR)/mmodel_analysis.cmi: E_ACSL_BFLAGS+= -w -60
+$(EACSL_PLUGIN_DIR)/mmodel_analysis.cmx: E_ACSL_OFLAGS+= -w -60
+
 ###############
 # Local Flags #
 ###############