make.defaults: handle negative incrementals in USE_EXPAND (530222)

Previously, USE_EXPAND variable settings in profile make.default only
supported positive incremental settings. This patch adds support for
negative settings like PYTHON_TARGETS="-python3_3".

X-Gentoo-Bug: 530222
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=530222