Vulkan tests fail on 32bit x86 systems
The Vulkan tests fail on 32bit x86 systems with errors such as:

```
[root@arch32-builder3 glad-2.0.6]# gcc -Wall -Wextra -Werror -Wsign-conversion -Wcast-qual -Wstrict-prototypes -ansi -pedantic -save-temps /build/glad/src/glad-2.0.6/test/c/compile/vulkan/alias/001/test.c -o /build/glad/src/glad-2.0.6/build/test -I/build/glad/src/glad-2.0.6/build/include /build/glad/src/glad-2.0.6/build/src/vulkan.c -ldl
In file included from /build/glad/src/glad-2.0.6/test/c/compile/vulkan/alias/001/test.c:9:
/build/glad/src/glad-2.0.6/build/include/glad/vulkan.h:4453:1: error: integer constant is too large for 'long' type [-Werror=long-long]      
 4453 | static const VkAccessFlagBits2 VK_ACCESS_2_SHADER_SAMPLED_READ_BIT = 4294967296;
```

This is because the tests are compiled with `-ansi`, but on 32bit x86 the only way to get a 64 bit integer is by using `long long`, which requires C99.

Passing `-std=c99` instead of `-ansi` resolves it. I am unsure as to how this should be properly resolved, as I assume you explicitly want to verify the code builds as C90 on other platforms.