Allow expansion of custom defined variables inside other variables

Currently if a user defined variable is included in another user defined variable it is replaced by an empty string. Instead we can replace it with the definition value of the definition. Since this is done in a while loop this continues until a we hit a fully defined variable, a predefined variable like $PATH or if no match is found the replacement is still an empty string. This useful for example if a custom runtime environment is configured in some PREFIX. Other variables like PATH or LD_LIBRARY_PATH can now depend on $PREFIX without needing to type the value of $PREFIX multiple times.