Warn users trying to generate a coverage report when not configured as such.

Makefile.am

@@ -86,6 +86,7 @@ reset-gcov:
 
 HTML_COVER_DIR=$(top_builddir)/coverage_html
 coverage-html: all
+if COVERAGE_ENABLED
 	test -e "`which lcov`" || (echo "lcov must be installed. See <http://ltp.sourceforge.net/coverage/lcov.php>." && false)
 	test -d "$(HTML_COVER_DIR)" || $(MKDIR_P) "$(HTML_COVER_DIR)"
 	lcov --rc lcov_branch_coverage=1 --directory $(top_builddir)/src --zerocounters
@@ -94,6 +95,9 @@ coverage-html: all
 	lcov --capture --rc lcov_branch_coverage=1 --no-external --directory $(top_builddir) --base-directory $(top_srcdir) --output-file "$(HTML_COVER_DIR)/lcov.tmp"
 	lcov --remove "$(HTML_COVER_DIR)/lcov.tmp" --rc lcov_branch_coverage=1 'test/*' 'ext/tinytest*' '/usr/*' --output-file "$(HTML_COVER_DIR)/lcov.info"
 	genhtml --branch-coverage -o "$(HTML_COVER_DIR)" "$(HTML_COVER_DIR)/lcov.info"
+else
+	@printf "Not configured with --enable-coverage, run ./configure --enable-coverage\n"
+endif
 
 # Avoid strlcpy.c, strlcat.c, aes.c, OpenBSD_malloc_Linux.c, sha256.c,
 # eventdns.[hc], tinytest*.[ch]