build-sys: Add a BSDMake target for converting pod files.

Signed-off-by: Moritz Buhl <mbuhl@openbsd.org>

man/man3/Makefile.am

@@ -62,7 +62,8 @@ man3_MANS += $(man3_MANS_generated)
 
 # Ensure the build is reproducible, for details see:
 # https://reproducible-builds.org/docs/source-date-epoch/
-%.3 : %.pod
+SUFFIXES: .pod .3
+.pod.3:
 	@if test -n "$$SOURCE_DATE_EPOCH"; then \
 		BUILD_DATE=$$(date +%F --utc --date="@$$SOURCE_DATE_EPOCH"); \
 	else \
@@ -70,7 +71,7 @@ man3_MANS += $(man3_MANS_generated)
 	fi; \
 	pod2man -r "libtpms" \
 		-c "" \
-		-n $(basename $@) \
+		-n $(<:.pod=) \
 		--date="$$BUILD_DATE" \
 		--section=3 $< > $@