Compiling port fails: header file cannot be found

[Moritz Schmitt]

Hello,

I am trying to install

/usr/ports/math/sage

by executing 'make install clean'. This fails with the following error message:

-----
Error building Sage.

The following package(s) may have failed to build:

package: zeromq-4.0.5
log file: /usr/ports/math/sage/work/sage-6.6/logs/pkgs/zeromq-4.0.5.log
-----

And zeromq-4.0.5.log contains additional information:

-----
(...)
ctx.cpp:39:20: fatal error: sodium.h: No such file or directory
 #include <sodium.h>
                    ^
compilation terminated.
-----

This means the header file sodium.h cannot be found. However, my system has it
in /usr/local/include. So it seems that the compiler needs to be told to look
for header files also in this directory. Is there a standard way to do this
when installing a port?

Thanks,
Moritz