Just #define _GNU_SOURCE and _USE_MATH_MACROS unconditionally

[sethk]

Remove definitions of M_PI from source.

• _USE_MATH_MACROS is required by MSVC when compiling against UCRT.
• _GNU_SOURCE is required for other functions that are extensions to

The macros in math.h are not part of POSIX, but are visible by default so long as you don't declare you are trying to be strictly POSIX-conformant by defining _POSIX_C_SOURCE or _XOPEN_SOURCE.

Tested on:

• macOS 11.7.9 (it's old, sorry)
  Apple clang version 13.0.0
  x86_64-apple-darwin20.6.0

• FreeBSD 14.2-RELEASE
  FreeBSD clang version 18.1.6
  x86_64-unknown-freebsd14.2

• Amazon Linux release 2 (Karoo) 4.14.336-253.554.amzn2.x86_64
  cc (GCC) 7.3.1 20180712 (Red Hat 7.3.1-17)
  x86_64-redhat-linux

• Windows 11 24H2:

  • MSYS2 c++ (GCC) 13.3.0
    x86_64-pc-msys
    msys-2.0.dll runtime

  • CLANG64 clang version 19.1.4
    x86_64-w64-windows-gnu
    ucrtbase.dll runtime

  • MINGW64 cc.exe (Rev2, Built by MSYS2 project)
    14.2.0 x86_64-w64-mingw32
    msvcrt.dll runtime

  • UCRT64 cc.exe (Rev2, Built by MSYS2 project)
    14.2.0 x86_64-w64-mingw32
    ucrtbase.dll runtime

  • Microsoft (R) C/C++ Optimizing Compiler Version 19.42.34433 for x64
    (Requires a few additional PRs that I will submit)

[sethk]

@alyssais and @kylophone , can you please review? I tried to simplify the defines here to also support MSVC on Windows, FreeBSD, and everything else imaginable without having to redefine M_PI in multiple places.

[kylophone]

Are _USE_MATH_MACROS and _GNU_SOURCE needed here for anything other than M_PI?

[sethk]

_USE_MATH_MACROS exposes M_PI when compiling against UCRT on Windows using MSVC or MinGW64 GCC. It has no effect anywhere else.
_GNU_SOURCE has no effect when using UCRT on Windows.
_GNU_SOURCE has no effect on FreeBSD.
_GNU_SOURCE has no effect on macOS.
_GNU_SOURCE exposes M_PI and strnlen() when compiling against glibc (on e.g. GNU/Linux, MSYS2).

strsep() is a BSD-ism and seems to be available everywhere except UCRT on Windows. It doesn't require _BSD_SOURCE on other systems. There is a local implementation of strsep() in libvmaf/tools/cli_parse.c, and a test for it in libvmaf/tools/meson.build, which may have been added for the sake of UCRT.

Per @alyssais ' last change to libvmaf/meson.build, as long as you don't declare strict POSIX or XOPEN conformance with the _POSIX_C_SOURCE and _XOPEN_SOURCE macros, then strnlen() and strsep() are available.