meson: stop setting -fPIE globally

Setting -fPIE globally can lead to miscompilations on certain
architectures.
This is caused by both -fPIE and -fPIC options being added to various
compilation commands. Only -fPIC is being recorded in the LTO options
section of the object. The gcc-8 LTO plugin merges -fPIC + -fPIE to
nothing. So, the compilations done by the plugin are not
position-independent and fail to link with -pie.

The simplest solution is to stop setting -fPIE globally and instead
using meson's b_pie=true option. This requires meson 0.49 or later.

Since we don't set this option in meson.build but leave it up to the
distro maintainer to set this option, do not bump the meson version
requirement.

Fixes: #10548