Add -std=gnu99 to CFLAGS on Windows

[yutannihilation]

I found AppVeyor keeps failing on tibble repo and it seems this is because of the absense of -std=gnu99: tidyverse/tibble#485

pkgbuild defines CFLAGS by itself, but Windows seems to need -std=gnu99 as the default is so.

PS C:\...> R.exe CMD config CFLAGS
-O2 -Wall -std=gnu99 -mtune=generic

PS C:\...> R.exe CMD config CXXFLAGS
-O2 -Wall -mtune=generic

PS C:\...> R.exe CMD config CXX11FLAGS
-O2 -Wall -mtune=generic

[jimhester]

Thanks for the PR! I decided to fix this in a different way instead.

[yutannihilation]

Oh, thanks for the fix!