Yes, it's in the cgo documentation
When the Go tool sees that one or more Go files use the special import "C", it will look for other non-Go files in the directory and compile them as part of the Go package. Any .c, .s, or .S files will be compiled with the C compiler. Any .cc, .cpp, or .cxx files will be compiled with the C++ compiler. Any .h, .hh, .hpp, or .hxx files will not be compiled separately, but, if these header files are changed, the C and C++ files will be recompiled. The default C and C++ compilers may be changed by the CC and CXX environment variables, respectively; those environment variables may include command line options.