build: ensure Visual Studio and Bison files don't get their line endings

corrupted even when using git-svn with automatic conversion.

.gitattributes

@@ -1 +1,8 @@
+# Visual Studio wants crlf
+*.sln eol=crlf
+*.csproj eol=crlf
+*.vcxproj eol=crlf
+# Vim doesn't like CRLF
 *.vim -text
+# Bison doesn't like CRLF
+external/*/share/bison/* -text