Fix C++ dependency warning

[jtcooper10]

Updating the C++ dependency list to look for SCons instead of GNU Make. The process for doing this is a little different from Make, since it's usually a Python package.

Changes Made

• Removed make from the list of C++ dependencies to check for, added scons
• Updated scons dependency check and call to look for the executable first, defaulting to the Python package if not found
• Updated warning in CSolver constructor to reference scons instead of make

Testing

Run test_check_cpp_support unit test in test_check_cpp_support.py or any explicit C++ solver test with:

• SCons installed as a Python package, e.g. python3 -m pip install scons (should build and run successfully)
• SCons installed externally, e.g. via a package manager or from the SCons download page (should build and run successfully)
• SCons not installed at all (should fail with the correct warning message)

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Fixes #923

[briandrawert]

When testing, be sure that model.run() will select the 'C' solver. Also verify that current develop does not do this.