[c/c++] Investigate $SYSROOT

[Mic92]

compiler/linker respect a variable called SYSROOT to set the default lookup path for includes/libraries. This might be handy for the devshell case.
Advantage:

• Rarely override in build systems unless they want to take over control, in which case the probably provide their own set of libraries/headers
• One environment variable for both header/libraries
• Also changes lookup path for libstdc++ and libc (Those can usually not just changed by using setting the INCLUDE_PATH to $dev/include, since they may contain subdirectories)

This still only affects the compile time. For runtime we still need flags like LD_LIBRARY_PATH to point to the right lib directory.

[Mic92]

Related to #102