build: don't enable PIE by default

build: don't enable PIE by default

PIE needs support from compiler, linker and dynamic linker. Even
when everything seems in place, it can be unreliable. On many ARM
toolchains, compiler and linker support is here but the dynamic
linker doesn't know how to handle that. It is also reported that
NetBSD/ppc or Solaris/x86 doesn't work correctly. Therefore, it is
too difficult to detect proper support at configure time (notably
with cross-compilation). Some OS (like OpenBSD, or Gentoo) come
with automatic PIE support. No need to do anyhing special for
them.

People wanting to enable PIE should now use --enable-pie.