Allow build to work without the system default compiler

[tst-ccamp]

The system default compiler is required for building the third party modules. Without it, the build fails here:

checking for c++... /home/jeff/earthenterprise/earth_enterprise/src/NATIVE-REL-x86_64/third_party/bin/g++
checking whether the C++ compiler (/home/jeff/earthenterprise/earth_enterprise/src/NATIVE-REL-x86_64/third_party/bin/g++  ) works... no
configure: error: installation or configuration problem: C++ compiler cannot create executables.

This is because third_party/bin/g++ is created with a hardcoded reference to /usr/bin/g++, even after running source /opt/rh/devtoolset-2/enable.

We need to allow the third party modules to be built with another C++ compiler (e.g. the devtoolset toolchain on EL 6). Also, remove the check for the default C++ compiler from earth_enterprise/src/SConstruct.

For more info and history see #1134