Add "--with-pic" to configure flags (specifically to fix i386)

After looking/comparing at the output of `./configure` for our i386 and amd64 builds against that of Debian's i386 build, our i386 build was the only one of the three to include `checking whether to force non-PIC code in shared modules... yes` (the other two were `no`).

Debian includes `--with-pic`, and we do not.

Looking at the upstream PHP code for this configure check (https://github.com/php/php-src/blob/313a56add0bacd05acc894a329ad1e68b583bc96/configure.ac#L241-L256), this makes total sense, and explains why i386 is the only architecture we see this issue on:

```m4
dnl Disable PIC mode by default where it is known to be safe to do so, to avoid
dnl the performance hit from the lost register.
AC_MSG_CHECKING([whether to force non-PIC code in shared modules])
case $host_alias in
  i?86-*-linux*|i?86-*-freebsd*)
    if test "${with_pic+set}" != "set" || test "$with_pic" = "no"; then
      with_pic=no
      AC_MSG_RESULT(yes)
    else
      AC_MSG_RESULT(no)
    fi
    ;;
  *)
    AC_MSG_RESULT(no)
    ;;
esac
```

This was probably already implied by our `CFLAGS` (and the compiler) on Alpine, but I figured it doesn't hurt to be explicit / consistent.

Author: tianon