Provide override for standard headers in C++

[robertu94]

Using Cxx on a system like Gentoo or some supercomputers with multiple
compilers installed in non-standard locations will result in Cxx not finding
the standard library. This change allows users to specify where to find
the system includes in a way that doesn't requiring forking Cxx to make
these changes. Since these systems are inherently non-standard, these
changes are unlikely to be fixed with a change to CollectLinuxHeaderPaths!
so providing anoverride is better.

This patch is an updated version of pull request #324 and #283 which no longer
compile with the current version of Cxx. It also serves as a solution to the issue
raised #315 that doesn't require a solution like VPetukhov suggested which
essentially forks Cxx for every user with these nonstandard locations.

[Gnimuc]

I think this is good to merge. Thanks for your contribution!

[schmrlng]

I see that this was just merged, but how do we feel about moving these overrides up one line, i.e.,

function collectAllHeaders!(headers, nostdcxx)
    for header in split(get(ENV, "CXXJL_HEADER_DIRS", ""), ":")
        isempty(header) || push!(headers, (header, C_System))
    end
    nostdcxx || collectStdHeaders!(headers)
    collectClangHeaders!(headers)
    headers
end

Otherwise I'm finding (at least on my machine) that headers in "/usr/include" still take precedence over custom "system" headers specified in ENV["CXXJL_HEADER_DIRS"].