[build][fix] Should only use `XI_CFLAGS` internally -

When `XI_EXTRA_CFLAGS` is used intenrally any external settings
will override it, e.g. running `make XI_EXTRA_CFLAGS=-I../../libfoo`
breaks all internal assignments to `XI_EXTRA_CFLAGS`.
Merge pull request #61 from xively/fix_usage_of_xi_extra_cflags

[build][fix] Should use only `XI_CFLAGS` internally -