Follow the existing practice and remove it under "distclean", same as
sh.config and Makefile.config
CLEAN := $(CLEAN) version.stamp notmuch-*.tar.gz.tmp
CLEAN := $(CLEAN) .deps
CLEAN := $(CLEAN) version.stamp notmuch-*.tar.gz.tmp
CLEAN := $(CLEAN) .deps
-DISTCLEAN := $(DISTCLEAN) .first-build-message Makefile.config sh.config
+DISTCLEAN := $(DISTCLEAN) .first-build-message Makefile.config sh.config sphinx.config
CPPCHECK_STAMPS := $(SRCS:%=.stamps/cppcheck/%)
.PHONY: cppcheck
CPPCHECK_STAMPS := $(SRCS:%=.stamps/cppcheck/%)
.PHONY: cppcheck