apis egl 1.0-1.3, glx 1.0 and vulkan 1.0 fail to compile
Using the glad 2, these apis fail to compile:

For egl:1.0-1.3, the error is: (the error disappears for egl:1.4-1.5)
```
/tmp/build/external/glad_egl_10/src/egl.c: In function ‘glad_egl_find_core_egl’:
/tmp/build/external/glad_egl_10/src/egl.c:781:33: error: ‘EGL_DEFAULT_DISPLAY’ undeclared (first use in this function); did you mean ‘EGL_BAD_DISPLAY’?
         display = eglGetDisplay(EGL_DEFAULT_DISPLAY);
                                 ^~~~~~~~~~~~~~~~~~~
                                 EGL_BAD_DISPLAY
/tmp/build/external/glad_egl_10/src/egl.c:781:33: note: each undeclared identifier is reported only once for each function it appears in
```

For glx:1.0, the error/warning is: (the error disappears for glx:1.1-1.4)
```
/tmp/build/external/glad_glx_10/src/glx.c: In function ‘glad_glx_has_extension’:
/tmp/build/external/glad_glx_10/src/glx.c:412:9: error: ‘GLAD_GLX_VERSION_1_1’ undeclared (first use in this function); did you mean ‘GLAD_GLX_VERSION_1_0’?
     if(!GLAD_GLX_VERSION_1_1)
         ^~~~~~~~~~~~~~~~~~~~
         GLAD_GLX_VERSION_1_0
/tmp/build/external/glad_glx_10/src/glx.c:412:9: note: each undeclared identifier is reported only once for each function it appears in
/tmp/build/external/glad_glx_10/src/glx.c:415:18: warning: implicit declaration of function ‘glXQueryExtensionsString’; did you mean ‘glXQueryExtension’? [-Wimplicit-function-declaration]
     extensions = glXQueryExtensionsString(display, screen);
                  ^~~~~~~~~~~~~~~~~~~~~~~~
                  glXQueryExtension
/tmp/build/external/glad_glx_10/src/glx.c:415:16: warning: assignment to ‘const char *’ from ‘int’ makes pointer from integer without a cast [-Wint-conversion]
     extensions = glXQueryExtensionsString(display, screen);
                ^
```

For vulkan:1.0, the errors are: (the error disappears for vulkan:1.1)
```
In file included from /tmp/build/external/glad_vulkan_10/src/vulkan.c:4:
/tmp/build/external/glad_vulkan_10/include/glad/vulkan.h:1070:40: error: ‘VK_FORMAT_G8B8G8R8_422_UNORM’ undeclared here (not in a function); did you mean ‘VK_FORMAT_B8G8R8A8_UNORM’?
     VK_FORMAT_G8B8G8R8_422_UNORM_KHR = VK_FORMAT_G8B8G8R8_422_UNORM,
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                        VK_FORMAT_B8G8R8A8_UNORM
/tmp/build/external/glad_vulkan_10/include/glad/vulkan.h:1071:40: error: ‘VK_FORMAT_B8G8R8G8_422_UNORM’ undeclared here (not in a function); did you mean ‘VK_FORMAT_B8G8R8A8_UNORM’?
     VK_FORMAT_B8G8R8G8_422_UNORM_KHR = VK_FORMAT_B8G8R8G8_422_UNORM,
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                        VK_FORMAT_B8G8R8A8_UNORM

...

/tmp/build/external/glad_vulkan_10/include/glad/vulkan.h:1743:53: error: ‘VK_OBJECT_TYPE_DESCRIPTOR_UPDATE_TEMPLATE’ undeclared here (not in a function); did you mean ‘VK_OBJECT_TYPE_DESCRIPTOR_SET_LAYOUT’?
     VK_OBJECT_TYPE_DESCRIPTOR_UPDATE_TEMPLATE_KHR = VK_OBJECT_TYPE_DESCRIPTOR_UPDATE_TEMPLATE,
                                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                     VK_OBJECT_TYPE_DESCRIPTOR_SET_LAYOUT
/tmp/build/external/glad_vulkan_10/include/glad/vulkan.h:1747:51: error: ‘VK_OBJECT_TYPE_SAMPLER_YCBCR_CONVERSION’ undeclared here (not in a function); did you mean ‘VK_STRUCTURE_TYPE_SAMPLER_YCBCR_CONVERSION_INFO_KHR’?
     VK_OBJECT_TYPE_SAMPLER_YCBCR_CONVERSION_KHR = VK_OBJECT_TYPE_SAMPLER_YCBCR_CONVERSION,
                                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                   VK_STRUCTURE_TYPE_SAMPLER_YCBCR_CONVERSION_INFO_KHR

```

See also this travis output: https://travis-ci.org/madebr/glad/builds/414842543 (ignore wgl:1.0)