Fix duplicate values in ENV vars and X_BUILD_FOR truncating BUILD_DEPENDS.

PKG_NOTES/PKG_ENV/CONFIGURE_ENV/MAKE_ENV/CO_ENV/QA_ENV were using += before the
double include check so any port using bsd.port.options.mk or bsd.port.pre.mk
would have duplicated items in these vars.

All ports would have their BUILD_DEPENDS truncated with X_BUILD_FOR.

PR: 198803 (exp run by antoine)
Differential Revision: https://reviews.freebsd.org/D2113
With hat: portmgr