How to add compile definitions to a pre-existing port?

[Cazadorro]

glfw broke centos/rhel 7 compatibility in a minor version change a while back, to fix it I had to add

target_compile_definitions(glfw PRIVATE _POSIX_C_SOURCE=199309L _POSIX_C_SOURCE=POSIX_REQUIRED_STANDARD _POSIX_SOURCE=POSIX_REQUIRED_STANDARD)

However, I want to use glfw3 from vcpkg now. The problem is that there doesn't appear to be a method to add compile definitions through pre-existing packages. The closest thing I could find was to make a custom port, but that straight up didn't work even on platforms where it should have (I just copied the directory for glfw3 from vcpkg and set the overlay port directory, caused some weird conflict), and again seemed to provide no avenue to apply these changes regardless.

Is there a way to unbreak glfw3 on rhel7 platforms using it through vcpkg?