Run check-spaces only when Perl is available

Also permit users to override the Perl variable with relative paths.

Makefile.am

@@ -181,11 +181,13 @@ coverage-html-full: all
 # Avoid strlcpy.c, strlcat.c, aes.c, OpenBSD_malloc_Linux.c, sha256.c,
 # tinytest*.[ch]
 check-spaces:
-	$(top_srcdir)/scripts/maint/checkSpace.pl -C \
+if USE_PERL
+	$(PERL) $(top_srcdir)/scripts/maint/checkSpace.pl -C \
 		$(top_srcdir)/src/common/*.[ch] \
 		$(top_srcdir)/src/or/*.[ch] \
 		$(top_srcdir)/src/test/*.[ch] \
 		$(top_srcdir)/src/tools/*.[ch]
+endif
 
 check-docs: all
 	$(PERL) $(top_builddir)/scripts/maint/checkOptionDocs.pl

configure.ac

@@ -195,7 +195,9 @@ AC_PROG_CPP
 AC_PROG_MAKE_SET
 AC_PROG_RANLIB
 
-AC_PATH_PROG([PERL], [perl])
+AC_ARG_VAR([PERL], [path to Perl binary])
+AC_CHECK_PROGS([PERL], [perl])
+AM_CONDITIONAL(USE_PERL, [test "x$ac_cv_prog_PERL" != "x"])
 
 dnl autoconf 2.59 appears not to support AC_PROG_SED
 AC_CHECK_PROG([SED],[sed],[sed],[/bin/false])