Rebuild the env.mk file when the Erlang/OTP has changed

We also rebuild the C code since C code tends to not stay
compatible between different Erlang versions.

Based on a patch by Slava Yurin.

plugins/c_src.mk

@@ -103,7 +103,7 @@ $(C_SRC_OUTPUT_FILE): $(OBJECTS)
 		$(LDFLAGS) $(if $(filter $(C_SRC_TYPE),shared),-shared) $(LDLIBS) \
 		-o $(C_SRC_OUTPUT_FILE)
 
-$(OBJECTS): $(MAKEFILE_LIST)
+$(OBJECTS): $(MAKEFILE_LIST) $(C_SRC_ENV)
 
 %.o: %.c
 	$(COMPILE_C) $(OUTPUT_OPTION) $<
@@ -125,13 +125,16 @@ clean-c_src:
 endif
 
 ifneq ($(wildcard $(C_SRC_DIR)),)
+ERL_ERTS_DIR = $(shell $(ERL) -eval 'io:format("~s~n", [code:lib_dir(erts)]), halt().')
+
 $(C_SRC_ENV):
 	$(verbose) $(ERL) -eval "file:write_file(\"$(call core_native_path,$(C_SRC_ENV))\", \
 		io_lib:format( \
 			\"# Generated by Erlang.mk. Edit at your own risk!~n~n\" \
 			\"ERTS_INCLUDE_DIR ?= ~s/erts-~s/include/~n\" \
 			\"ERL_INTERFACE_INCLUDE_DIR ?= ~s~n\" \
-			\"ERL_INTERFACE_LIB_DIR ?= ~s~n\", \
+			\"ERL_INTERFACE_LIB_DIR ?= ~s~n\" \
+			\"ERTS_DIR ?= $(ERL_ERTS_DIR)~n\", \
 			[code:root_dir(), erlang:system_info(version), \
 			code:lib_dir(erl_interface, include), \
 			code:lib_dir(erl_interface, lib)])), \
@@ -143,6 +146,10 @@ distclean-c_src-env:
 	$(gen_verbose) rm -f $(C_SRC_ENV)
 
 -include $(C_SRC_ENV)
+
+ifneq ($(ERL_ERTS_DIR),$(ERTS_DIR))
+$(shell rm -f $(C_SRC_ENV))
+endif
 endif
 
 # Templates.

plugins/ci.mk

@@ -32,7 +32,7 @@ ci_verbose = $(ci_verbose_$(V))
 
 define ci_target
 ci-$1: $(KERL_INSTALL_DIR)/$2
-	$(verbose) $(MAKE) --no-print-directory clean distclean-c_src-env
+	$(verbose) $(MAKE) --no-print-directory clean
 	$(ci_verbose) \
 		PATH="$(KERL_INSTALL_DIR)/$2/bin:$(PATH)" \
 		CI_OTP_RELEASE="$1" \