Gentoo/Portage

Enviroment flags only (CFLAGS, LINGUAS, etc)
 x11-libs/qt-webkit nodebug.conf

 CFLAGS="-march=native -Os -pipe -msse3" CXXFLAGS="${CFLAGS}"

Uses the same format as make.conf(5)

Alternative
Create in  with the custom environment.

= Environmental variables =

EXTRA_ECONF / EXTRA_EMAKE
Pass extra flags to econf (configure) and emake (make): EXTRA_ECONF="--disable-silent-rules" emerge -v1 foobar