Accommodate old Vim (<7.4) versions using perl6.vim

Running "make install" will now remove the regex optimization from the
syntax file if the user has an old Vim.

For those who try to run an unmodified perl6.vim, we now print an error
when the syntax file is loaded.

Makefile

@@ -10,7 +10,7 @@ default: preproc
 dirs:
 	mkdir -p $(FTPLUGIN) $(INDENT) $(SYNTAX) $(TOOLS)
 
-install: dirs
+install: dirs fix_old_vim
 	cp ftplugin/*.vim    $(FTPLUGIN)/
 	cp indent/*.vim      $(INDENT)/
 	cp syntax/*.vim      $(SYNTAX)/
@@ -63,3 +63,7 @@ try-tiny: contrib_syntax
 
 preproc:
 	tools/preproc.pl syntax/perl6.vim.pre > syntax/perl6.vim
+
+# this gets rid of a regex optimization introduced in Vim 7.4
+fix_old_vim:
+	expr `vim --version|head -n1|grep -Poh '\d\.\d'|head -n1` \< 7.4 >/dev/null && sed -i 's/\\@[0-9]\+/\\@/g; /version < 740/d' syntax/perl6.vim; true

syntax/perl6.vim

@@ -33,6 +33,8 @@
 " To enable folding:
 "   let perl6_fold=1
 
+if version < 740 | throw "perl6.vim uses regex syntax which Vim <7.4 doesn't support. Try 'make fix_old_vim' in the vim-perl repository." | endif
+
 " For version 5.x: Clear all syntax items
 " For version 6.x: Quit when a syntax file was already loaded
 if version < 600

syntax/perl6.vim.pre

@@ -33,6 +33,8 @@
 " To enable folding:
 "   let perl6_fold=1
 
+if version < 740 | throw "perl6.vim uses regex syntax which Vim <7.4 doesn't support. Try 'make fix_old_vim' in the vim-perl repository." | endif
+
 " For version 5.x: Clear all syntax items
 " For version 6.x: Quit when a syntax file was already loaded
 if version < 600