Diana / metlibs use the old, unsupported UDUNITS library, not the new UDUNITS-2 library. Unfortunately, converting from one to the other is not trivial, so we're stuck with the old library—for now.

First, get the latest source (1.12.9) from http://www.unidata.ucar.edu/downloads/udunits/index.jsp and extract it in a convenient place.

Building UDUNITS is not straightforward. Its autotools infrastructure is old, and its configure script does not use the correct autoconf idiom for locating yacc, so you have to specify it manually; moreover, it fails to recognize g77, and uses POSIX features without defining the appropriate _XOPEN_SOURCE macro.

The configure command line is a bit of a mess:

$ cd src
$ LD_YACC=-ly LDFLAGS=-no-undefined CPPFLAGS='-Df2cFortran -D_POSIX_MAX_INPUT=255' FC=g77 ./configure --prefix=/opt/udunits

UDUNITS includes Perl modules and man pages which require additional tools to build, so we'll just manually build and install the library and binary:

$ cd lib
$ make installed_library installed_headers
$ cd ../udunits
$ make installed_program