Undefined vec.h when compiling simgear

[sathish]

Hi,
Iam trying to compile the sourcecode of FG 2017.3.1 along with simgear and Osg ...
While compiing the Simgear's Trimesh.h file inside dem folder, its throwing me an error...
"Undefined reference to vec.h,Box.h" ,where can i find the definition of this....

Anybody help please...