In the README in the GNU units source code (2.21):

  203: Ideas the future (may or may not happen):
  204:
  205:  * Bundle up the units conversion stuff into a library. 

At the end of units.c, too:

  6280: /* NOTES:
  6281: 
  6282: mymalloc, growbuffer and tryallunits are the only places with print
  6283: statements that should (?) be removed to make a library.  How can
  6284: error reporting in these functions (memory allocation errors) be
  6285: handled cleanly for a library implementation?  
  6286: 
  6287: Way to report the reduced form of the two operands of a sum when
  6288: they are not conformable.
  6289: 
  6290: Way to report the required domain when getting an domain error.
  6291:
  6292: */

My prowling of the source code is probably uninformed because I'm not a C expert by any means but it seems like you could include units.h and use many of the useful functions in another program.

You might also be interested in boost::units.

It implements units using a ton of templates and "compile-time metaprogramming". [0]

The nice thing is that it has no runtime overhead as all templates are "compiled away". The disadvantage is that when you make a mistake against units, it results in a "template compiler error explosion".

[0]https://www.boost.org/doc/libs/1_65_0/doc/html/boost_units.h...

Per another thread it sounds like Frink is what you want.

https://frinklang.org/

A program is easier to use. You do not need to link to it, just run it! Do you have trouble running programs?