Include src/include/wiredtiger_ext.h is problematic

To include wiredtiger_ext.h, I can specify -Iwiredtiger/src/include. Problem is that src/include has error.h which overrides /usr/include/error.h.

Since wiredtiger_ext.h is used by external applications, it should be where wiredtiger.h is.

I understand applications should use the installed files in /usr/local but during development, tests may need to point to where wiredtiger source path for include and library. Does that make sense?