Make:Allow for extension in tool names.

In some build environments executable programs have a specific file
extension.  The value of BIN_EXT is appended to the relevant tool file
names to allow for this.
The value of BIN_EXT is set, where appropriate, by the build environment
specific make helper (to .exe for Windows build environments).

.gitignore is updated to hide the new (.exe) files.

Change-Id: Icc32f64b750e425265075ad4e0dea18129640b86
diff --git a/make_helpers/cygwin.mk b/make_helpers/cygwin.mk
index f99be13..e69c0b1 100644
--- a/make_helpers/cygwin.mk
+++ b/make_helpers/cygwin.mk
@@ -37,4 +37,7 @@
 
     include ${MAKE_HELPERS_DIRECTORY}unix.mk
 
+    # In cygwin executable files have the Windows .exe extension type.
+    BIN_EXT := .exe
+
 endif