Fixed build script that called C++ compiler instead of C compiler

[onion108]

The original build script uses $CXX (which is a variable that usually stands for a C++ compiler) to detect a C compiler, which may cause problems in some cases (for example my $CXX is set to clang++ (actually /opt/homebrew/opt/llvm/bin/clang++ but it doesn't matter), and when trying to pass -std=c11 to clang++ it will complain about that it cannot be used to compile C++. It could be solved by passing -x c to this specific compiler but it would be better to use C compiler though.).