Simplify alloca.h include condition

[RyuKojiro]

And make it way more portable. While alloca isn't standardized, glibc is the only place that uses alloca.h. This is the only thing keeping imgui from building on NetBSD.

[ocornut]

Hello, Thanks for your PR. Could you clarify which system+compiler did you test that with?

[ocornut]

Now tested successfully with:

• Windows + MSVC 2010
• Windows + MSVC 2015
• PS4
• Xbox One
• Switch
• Linux + GCC
• Linux + Clang
• OSX + GCC
• OSX + Clang

Merging, thanks!

[krytarowski]

alloca() in <alloca.h>-only is SunOS specific
alloca() in <stdlib.h> and <alloca.h> is available on Windows, FreeBSD, OpenBSD, Darwin
alloca() in <stdlib.h> only in NetBSD specific

[ocornut]

Hello @krytarowski,

I don't know how those statement relate to to use of GLIBC. Did the change break anything for you on those platforms?

[krytarowski]

#elif !defined(__GLIBC__) -> #elif !defined(__GLIBC__) && !defined(__sun) to fix SmartOS

[ocornut]

Made that change now @krytarowski @RyuKojiro
Also ref #276