The CPPFLAGS, CFLAGS, CXXFLAGS and LDFLAGS differ between the build phase
and the install phase. It's only a minor difference but may still
influence packages that use these flags at install time, even though they
shouldn't.
For now just document that the flags differ.
build and install phase. MAKE_ENV needs to remain the same for
packages that use scons. Otherwise, in the install step, scons will
take the new MAKE_ENV to mean that the build action changed, and it will
rebuild the package with the wrong configuration and install that.
