see https://bugs.gentoo.org/868000 and https://bugs.gentoo.org/980185
The output file was previously passed as an argument.
Using '-o' explicitly sets the output file.
Pass -x c to preprocess files with non-standard extensions (.pre)
--- a/man/Makefile.am
+++ b/man/Makefile.am
@@ -189,7 +189,7 @@ MANS_COMMON = \
 	manpage-common-legacy-output.man
 
 $(MANS_CPP): $(MANS_CPP:%=%.pre)
-	$(CPP_FOR_BUILD) -nostdinc -w -C -P -I. -I$(top_srcdir)/man $@.pre $@
+	$(CPP_FOR_BUILD) -x c -nostdinc -w -C -P -I. -I$(top_srcdir)/man $@.pre -o $@
 
 CLEANFILES = $(MANS_CPP) *~
 DISTCLEANFILES = $(MANS_CPP) $(MANS_CPP:%=%.pre) .deps/*.P