xpp: perl man pages: Perl may be upper case

In file's output Perl scripts may be 'awk or perl script',
'/usr/bin/perl script' or 'Perl script' (with some minor variants). The
latter was not detected by current code and thus perld man pages were
not created.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>

xpp/Makefile.am

@@ -24,7 +24,7 @@ if PERL
 SUBDIRS		+= perl_modules
 
 %.8: %
-	@if file "$^" | cut -d: -f2 | grep -q perl; then \
+	@if file "$^" | cut -d: -f2 | grep -q -iw perl; then \
 		if pod2man --section 8 $^ > $@; then \
 			echo "  GEN      $@"; \
 		else \