Enable the GNU extensions

[BSzili]

I'd like to get this nastiness:
https://github.com/EasyRPG/liblcf/blob/master/src/writer_xml.cpp#L12
The problem is what -std=c++11 defines __STRICT_ANSI__ which causes a lot of functions to go undeclared on AmigaOS4. I made this a separate PR in case it causes any problems.

[Ghabry]

Is it possible to update newlib?
They fixed these macros 4 years ago: bminor/newlib@feec81e

[BSzili]

I'm not part of the OS4 developer team, so I can't update it. I want to avoid hacking the SDK headers, because it would create a big mess, especially for others who would like to build liblcf, I might break other software, etc.
Unfortunately this seems to be the only way to be compatible with the official OS4 SDK without resorting to workarounds I added previously.

[Ghabry]

Well can't comment further on this as I don't know if gnu++14 has any disadvantages for us.

[fdelapena]

In case we accidentally use some GNU extension in code, clang MSVC automated builds not based on autotools/cmake might fail, so maybe it's safe.

[fmatthew5876]

I believe the only downside is that we if we use some gnu extension by mistake, non-gnu builds will fail. But then we'll see it in jenkins.

So I'm fine with this.