The pkgconfig file tk.pc.in shipped with tk sources [1] uses an undefined autotools tag @TK_LIBS@ which leads to the installed pkgconfig file tk.pc having the wrong linker flags:
Libs: -L${libdir} @TK_LIBS@

The correct value for the Libs tag would likely be something like
Libs: -L${libdir} -ltk8.6

This is fixed by the attached patch. This replaces the undefined @TK_LIBS@ by the token @TK_LIB_FLAG@, which generates the .pc file with the correct linker flag upon installation by autotools.

[1] See here: