Feature Request: Make C-header files compatible with C++

In a previous email thread, @xiaoxiang781216, @pkarashchenko, and I discussed making C header files compatible with c++. This is necessary so a C++ application can `#include` C headers w/o compilation issues. See this PR where I wanted to include some segger RTTI files in my c++ app and had to rename some c++ identifiers.
The conclusion was this:
> We can add a simple test like this:
> 1. Create a context target in Makefile to generate a .cxx file which
list and include all header files in nuttx/include
> 2. Add the generated file to CXXSRCS
> If we just want to ensure the public nuttx header files are compatible with
c++ syntax.

I can make the changes to make the header files c++-compatible. I just want to document the feature request here for posterity