All the tests for compilers (AC_PROG_CC, AC_PROG_CXX,
AC_PROG_F77) define the output variable EXEEXT based on
the output of the compiler, typically to the empty string if
Posix and ‘.exe’ if a DOS variant.
They also define the output variable OBJEXT based on the
output of the compiler, after .c files have been excluded, typically
to ‘o’ if Posix, ‘obj’ if a DOS variant.
If the compiler being used does not produce executables, the tests fail. If the executables can't be run, and cross-compilation is not enabled, they fail too. See Manual Configuration, for more on support for cross compiling.