Add PIE option to Makefile to build position independent executables

[icota]

As of version 5.0 Android requires all dynamically linked executables to support PIE. This allows programs to be loaded at a different addresses, making it harder for attackers to target.

Enable with PIE=1
Somewhat related to #653

[practicalswift]

Concept ACK

Hardening is good

[ZmnSCPxj]

I begin to wonder if we should do something closer to:

# User can add flags here on "make" command line
# e.g. make CFLAGS="-fPIC -fPIE"
CFLAGS =

MY_CFLAGS = # whatever is currently in CFLAGS in current version

COMPILE = $(CC) $(MY_CFLAGS) $(CFLAGS)
.c.o:
        $(COMPILE) $< -o $@

And similar for linking.... hmmm.

Thoughts?

[icota]

I'm fine either way really. Did it this way cause DEV_CFLAGS were already in there. Didn't want to mess with the way things were done I guess.

[cdecker]

utACK 3c7cf2a

@icota are you trying to get c-lightning running on android? That'd be huge 😃

[icota]

@cdecker I have it running on my phone already. A couple more PRs and we will be good to go. 🍾

[rustyrussell]

ACK 3c7cf2a