bam should honour system toolchain

[hasufell]

In order for packages that use bam to be correctly packaged with the distribution compiler which may be prefixed (e.g. x86_64-pc-linux-gnu-gcc instead of gcc), one has to do hackery like (example for teeworlds):

    cat << __EOF__ > "${WORK}/settings.lua"
    function addSettings(settings)
        settings.optimize = 0
        settings.cc.exe_c = "${CC}"
        settings.cc.exe_cxx = "${CXX}"
        settings.cc.flags_c:Add("${CFLAGS}")
        settings.cc.flags_cxx:Add("${CXXFLAGS}")
        settings.link.exe = "${CXX}"
        settings.link.flags:Add("${LDFLAGS}")
    end
__EOF__

    edo sed -i \
        -e '/if family == "unix" then/a \\t\tdofile("settings.lua") addSettings(settings)' \
        bam.lua

Instead, pretty much every major build system honors variables like CC when they are set in the environment or at least gives you very easy means to change them without changing any file.

So, bam should either

1. allow to set these things via environment variables
2. allow to set these variables via bam --some-switch invocation

Hacking on every possible bam.lua file just to get the toolchain right is not a solution.

[matricks]

Fixed by 2e65531