[Makefile] Fixes headers target to do the correct thing in internal mode

Integration within main distribution might work at some non-so-far-way
point in the future after all.

src/plugins/e-acsl/Makefile.in

@@ -3,7 +3,7 @@
 #  This file is part of the Frama-C's E-ACSL plug-in.                    #
 #                                                                        #
 #  Copyright (C) 2012-2016                                               #
-#    CEA (Commissariat à l'énergie atomique et aux énergies              #
+#    CEA (Commissariat  l'nergie atomique et aux nergies              #
 #         alternatives)                                                  #
 #                                                                        #
 #  you can redistribute it and/or modify it under the terms of the GNU   #
@@ -300,12 +300,18 @@ e-acsl-distrib-check:
 
 headers::
 	@echo "Applying Headers..."
-	headache -c license/headache_config.txt -h license/CEA_LGPL \
-		*.ml *.mli \
-		Makefile.in configure.ac \
-		share/e-acsl/*.[ch] share/e-acsl/*/*.[ch] scripts/*.sh
-	headache -c license/headache_config.txt -h license/SPARETIMELABS \
-		share/e-acsl/e_acsl_printf.h
+	headache -c $(EACSL_PLUGIN_DIR)/license/headache_config.txt \
+                 -h $(EACSL_PLUGIN_DIR)/license/CEA_LGPL \
+	         $(EACSL_PLUGIN_DIR)/*.ml \
+                 $(EACSL_PLUGIN_DIR)/*.mli \
+	         $(EACSL_PLUGIN_DIR)/Makefile.in \
+                 $(EACSL_PLUGIN_DIR)/configure.ac \
+	         $(EACSL_PLUGIN_DIR)/share/e-acsl/*.[ch] \
+                 $(EACSL_PLUGIN_DIR)/share/e-acsl/*/*.[ch] \
+                 $(EACSL_PLUGIN_DIR)/scripts/*.sh
+	headache -c $(EACSL_PLUGIN_DIR)/license/headache_config.txt \
+                 -h $(EACSL_PLUGIN_DIR)/license/SPARETIMELABS \
+		  $(EACSL_PLUGIN_DIR)/share/e-acsl/e_acsl_printf.h
 
 ################
 # Generic part #