Build: Change --enable-ifunc handling.

Build: Change --enable-ifunc handling.

Some compilers support __attribute__((__ifunc__())) even though the
dynamic linker does not. The compiler is able to create the binary
but it will fail on startup. So it is not enough to just test if
the attribute is supported.

The default value for enable_ifunc is now auto, which will attempt
to compile a program using __attribute__((__ifunc__())). There are
additional checks in this program if glibc is being used or if it
is running on FreeBSD.

Setting --enable-ifunc will skip this test and always enable
__attribute__((__ifunc__())), even if is not supported.