Compile C using gcc for Windows in Ubuntu

1. Install mingw cross-compiler: 

apt install mingw-w64

2.a. Compile for 32-bit executable:

i686-w64-mingw32-gcc -o main32.exe main.c

2.b. Compile for 64-bit executable:

x86_64-w64-mingw32-gcc -o main64.exe main.c