Gl3w generate a C file but is using C++ stl

[ahmidou]

ofiwg/libfabric#7041 (comment)

We made this change very recently in microsoft/STL#2148 which forbids including standard library headers from C programs.

I can't build with gl3w anymore in VS 2022 17.1 and MSVC toolset 143

a simple fix would be to change the c extension to cpp