arm: define PIC if the compiler uses pic by default

Necessary for the way fedora enables `-fPIE` by default for package
builds. Fixes #221.
12 jobs for master in 4 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #243570
amd64 debian
build-debian

00:00:31

passed #243574
debian aarch64
build-debian-aarch64

00:01:29

passed #243575
debian aarch64
build-debian-aarch64-clang-5

00:01:06

passed #243571
amd64 debian
build-debian-static

00:00:34

passed #243577
debian aarch64
build-debian-werror

00:00:33

passed #243576
macos
build-macos

00:00:28

passed #243572
win32
build-win32

00:00:28

passed #243573
win64
build-win64

00:00:37

 
  Test
passed #243578
amd64 debian
test-debian

00:00:42

passed #243579
amd64 debian
test-debian-asan

00:02:06

passed #243580
amd64 debian
test-debian-msan

00:00:57

passed #243581
amd64 debian
test-debian-ubsan

00:01:17