Code: Fixes include guard condition

[am11]

With this change, we can compile src/c99func.c without any extra condition in down-streams' build process.

[saper]

Well, the whole point of the c99func.c is not to compile that file when not needed... you can leave ifdefs if you Iike but I'd leave msbuild conditional rules in place.

[am11]

Yeah I remember the point of c99func.c. However, the point of this PR is to save every downstream from checking the condition to be able to build with both VS2013 and VS2015. See https://github.com/am11/node-sass/blob/request-ci-artifacts/src/libsass.gyp#L82-L84, there is no decent condition to check MSC_VER there and this approach saved me sometime and hopefully will save others' as well.

[saper]

That is why I think we can keep MSBuild rule and leave conditionals inside. I will deal with libsass.gyp once our gyp fully supports vs2015.
I want to make sure we don't get any linker errors when not linking c99func.obj

[am11]

I found this guy trying to figure out a way to detect toolset version in gyp: http://stackoverflow.com/q/26311665/863980 (received no answer).
Then tested with VS2013 (on AppVeyor) and VS2015 (both locally and on AppVeyor) with this change, there is no linker error so far; compilation succeeds (only some tests are failing atm).

[saper]

Here it goes: saper/node-sass@edf7127 - can you check that with VS2015 if c99func.c gets excluded from the build? This is sass/node-sass#1040

[am11]

Yup c99func gets excluded from VS2015, when ps: $env:GYP_MSVS_VERSION=2015. Nicely done!

BTW, AppVeyor is also building with VS2015, but the file pangyp was emitting VS2013 proj files, with c99func. I have sent a PR on your fork.