Have you tried something like

(add-before 'configure 'set-ldflags
   (lambda _
     (substitute* "tecmake.mak"
       (("STDLDFLAGS := -shared" all) (string-append all " 
-Wl,-rpath="#$output "/lib")))))

not tested though.

Also it looks like a cmake project, why are you not using the 
cmake-build-system?

Cheers,

-- 
Reza Housseini

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