routing extension additions break CESM driver compiling

[dgergel]

When using the current develop branch, I get an error while compiling the CESM driver because the rout_extension_name.h file cannot be found:

/p/home/gergel/rasm_vic5/models/lnd/vic/vic/drivers/shared_image/include/vic_driver_shared_image.h(30): catastrophic error: cannot open source file "rout_extension_name.h"
  #include <rout_extension_name.h>
                                  ^

compilation aborted for /p/home/gergel/rasm_vic5/models/lnd/vic/vic/drivers/shared_image/src/alloc_atmos.c (code 4)
gmake: *** [alloc_atmos.o] Error 4
gmake: *** Waiting for unfinished jobs....
In file included from /p/home/gergel/rasm_vic5/models/lnd/vic/vic/drivers/shared_image/src/alloc_veg_hist.c(27):
/p/home/gergel/rasm_vic5/models/lnd/vic/vic/drivers/shared_image/include/vic_driver_shared_image.h(30): catastrophic error: cannot open source file "rout_extension_name.h"
  #include <rout_extension_name.h>
                                  ^

compilation aborted for /p/home/gergel/rasm_vic5/models/lnd/vic/vic/drivers/shared_image/src/alloc_veg_hist.c (code 4)
gmake: *** [alloc_veg_hist.o] Error 4```

The `~/extensions` paths need to be added to the CESM driver build routines so that they are able to be found at compile-time. 

[dgergel]

Closed via #756