As another user reported here: http://www.nabble.com/--mesa3d-Bugs-1844243---GLX-header-definition-t4944614.html

In the header glx.h, all functions declarations are not encapsulated into the consistent definitions (e.g. "#ifndef GLX_VERSION_1_x"). All defines are set at the beginning of the header file ("#define GLX_VERSION_1_1 1", etc) 

In a mixed environment where a project uses GLEW but depends on a library that includes glx.h, they will get this error:

/usr/include/GL/glx.h:202: error: declaration of C function 'void glXCopyContext(Display*, __GLXcontextRec*, __GLXcontextRec*, long unsigned int)' conflicts with
/usr/include/GL/glxew.h:118: error: previous declaration 'void glXCopyContext(Display*, __GLXcontextRec*, __GLXcontextRec*, GLuint)' here

The reason being that the types don't agree for the last parameter. I've filed a bug with GLEW to fix this on their end:

http://sourceforge.net/tracker/index.php?func=detail&aid=1845264&group_id=67586&atid=523274

But I think it also makes sense to encapsulate the functions in glx.h, thus this report.