Preserve intermediate .c files in coverage mode

The introduction of the .y -> .c pattern rule causes some .c files such
as bootparse.c to be considered intermediate files in the .y -> .c -> .o
rule chain, which make would automatically delete.  But in coverage
mode, the processing tools such as genhtml need those files, so mark
them as "precious" so that make preserves them.

src/Makefile.global.in

@@ -329,6 +329,8 @@ GENHTML = @GENHTML@
 ifeq ($(enable_coverage),yes)
 # ccache loses .gcno files
 export CCACHE_DISABLE = 1
+# preserve intermediate .c files for genhtml
+.PRECIOUS: %.c
 endif
 
 # Feature settings