Testing for empty input files: streambuf::in_avail() isn't robust accross platforms

In ./src/macro/Driver.cc, streambuf::in_avail() == 0 is used to test whether the input file is empty. Using the cross-compiler of Julia infrastructure https://github.com/JuliaPackaging/Yggdrasil and BinaryBuilder.jl, under macOS, in_avail() return 0 just after the opening of the file even if the file isn't empty. This behavior can be demonstated with the attached example test.cc[test.cc]

It isn't completely clear to me whether the cross-compiler used by BinaryBuilder.jl is buggy or whether one shouldn't rely on in_avail() to test whether a file is empty.

For DynareJulia, I patch the current source code with modfile.peek() == std::istream::traits_type::eof() but to do so I have to remove const on modfile. Here is the patch file patches.patch

Furthermore, testing for an empty input file shouldn't be done in Driver.cc but higher up in DynareMain.cc. Currently if the user supplies an empty *.mod file, macroExpandModFile() returns without attempting to parse the file for macros, but the preprocessor fails with a cryptic error when the second stage lexer attempts to read an empty string.