Compile with --pic on Linux if linker is gcc

[Theodus]

Previously, users on platforms where gcc enables PIE by default were
required to compile with the --pic flag. This change automatically
compiles with --pic on Linux and FreeBSD when the linker is gcc.

Resolves #1811

[SeanTAllen]

Can you explain what is happening here @Theodus. I'm looking at the commit and don't understand how it addresses the --pic issue.

[Theodus]

@SeanTAllen on LInux and FreeBSD the opt.pic is set to true if the linker is cc or gcc: https://github.com/ponylang/ponyc/pull/1819/files#diff-35dfc63f568c5616abecb849fd7abdd3R373
This requires the environment variable CC to be checked before genexe.

[SeanTAllen]

I think we need a more robust test for "they are using gcc" than

if(strcmp(opt.linker, "gcc") == 0 || strcmp(opt.linker, "cc") == 0)

I don't know what is possible but a basic string comparison seems very brittle.

[Theodus]

I agree but I don't know what else could be done. I would use the output of --version, but with cc it will not specify that it is gcc, it will just give the same version info and copyright notice:

theodus@blade /mnt/c/Users/theo/dev/ponyc (issue-1811) $ cc --version
cc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. 

[SeanTAllen]

I think we should discuss at sync and get feedback because I as well don't know what else we can do. Unless we start doing "configure" style probing.

[Theodus]

After discussing on the sync meeting we have decided not to do this.