GIT: adjust gitignore

- ignore top-level /build and /platforms directories

.gitignore

@@ -39,8 +39,12 @@ solaris*Gcc*/
 SunOS*Gcc*/
 platforms/
 
-# Reinstate wmake/rules that might look like build directories
-!wmake/rules/*/
+# Top-level build directories
+/build/
+/platforms/
+
+# Reinstate wmake rules that might look like build directories
+!/wmake/rules/*/
 
 # doxygen generated documentation
 doc/Doxygen/html
@@ -62,6 +66,7 @@ doc/Doxygen/DTAGS
 # Source packages - anywhere
 *.tar.bz2
 *.tar.gz
+*.tar.xz
 *.tar
 *.tgz
 *.gtgz