Merge branch 'jc/sparse-error-for-developer-build'

"make DEVELOPER=1 sparse" used to run sparse and let it emit
warnings; now such warnings will cause an error.

* jc/sparse-error-for-developer-build:
  Makefile: enable -Wsparse-error for DEVELOPER build

config.mak.dev

@@ -1,5 +1,6 @@
 ifeq ($(filter no-error,$(DEVOPTS)),)
 DEVELOPER_CFLAGS += -Werror
+SPARSE_FLAGS += -Wsparse-error
 endif
 ifneq ($(filter pedantic,$(DEVOPTS)),)
 DEVELOPER_CFLAGS += -pedantic