Using different compilers does not always go to dependencies

[KrisThielemans]

In bash, you can normally say

CC=gcc-5 CXX=g++-5 cmake .

to use a non-default compiler. However, these settings of CC do not get passed on to (some?) dependencies with the superbuild. It currently only works if you do

export CC=gcc-5
export CXX=g++-5
cmake .

[KrisThielemans]

This might help for the CMake projects (but not for configure etc)

http://stackoverflow.com/questions/32744788/forwarding-current-compiler-to-externalproject?rq=1

[KrisThielemans]

This should no longer be a problem for CMake projects by using mark_as_superbuild. So I guess only configure projects affected. The suggested solution should work, so we'll postpone.

[KrisThielemans]

The suggested solution of setting CC and CXX does not work for boost as Boost.Build ignores this and uses the system gcc anyway. It should be solvable by creating a user-config.jam, see an answer by Dave Riedel here

I have several different versions of gcc I need to work with so,
I have done this for a long time by modifying my user-config.jam as follows:
using gcc : : /<path to custom gcc>/bin/g++ : <linkflags>"-Wl,-rpath
-Wl,/<path to custom gcc>/lib64" ;

You need the rpath so that any shared libraries will find the correct
gcc runtime libraries. 

This would conceivably be possibly to do via CMake here, but it isn't trivial to get the complete and correct paths. The linkflags are presumably gcc specific as well.

we might still have to run ./b2 --toolset=gcc.

Boost bootstrap.sh seems to have a special toolset cc that might use CC environment variable, but @evgueni-ovtchinnikov and I couldn't get that to work.

[KrisThielemans]

Specifying compilers is actually not enough. We need to use the same C++ versions as well. See #161