The docs say:

Shader compiler support is optional, and thus must be queried before use by calling glGet with argument GL_SHADER_COMPILER. glShaderSource, glCompileShader, glGetShaderPrecisionFormat, and glReleaseShaderCompiler will each generate GL_INVALID_OPERATION on implementations that do not support a shader compiler. Such implementations instead offer the glShaderBinary alternative for supplying a pre-compiled shader binary.

Which to me says that if it doesn't support glShaderSource, then you can safely fall back to glShaderBinary. If glShaderBinary is not supported, then you can safely fall back to glShaderSource. I don't think a valid implementation can offer no support for either (they must, as far as I'm aware, support at least one, if not both).