getting build error in /wex/src/gleasy.cpp

I'm trying to build the sdk and builder all four projects in one but I'm getting this in /wex/src/gleasy.cpp. which says 
 error: ‘glXGetProcAddress’ was not declared in this scope
         glWindowPos2f = (PFNGLWINDOWPOS2FPROC)glXGetProcAddress((const GLubyte*)"glWindowPos2f");
                                                                                                ^
wex/src/CMakeFiles/wex.dir/build.make:380: recipe for target 'wex/src/CMakeFiles/wex.dir/gleasy.cpp.o' failed

Please suggest how can I fix it. 

Hi Om Prakash,

I assume you are following the instructions here: github.com/NREL/SAM/wiki .

What operating system are you using?

Best regards,
Paul.