build with gssapi and without unit test fail

when used --with-gssapi and without --with-gtest-source kea still try to build nsupdate needed for tests:

make[7]: Leaving directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig/libloadtests'
Making all in tests
make[7]: Entering directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig/tests'
Making all in .
make[8]: Entering directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig/tests'
  CXXLD    nsupdate
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
make[8]: *** [Makefile:683: nsupdate] Error 1
make[8]: Leaving directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig/tests'
make[7]: *** [Makefile:897: all-recursive] Error 1
make[7]: Leaving directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig/tests'
make[6]: *** [Makefile:809: all-recursive] Error 1
make[6]: Leaving directory '/home/wlodek/kea/premium/src/hooks/d2/gss_tsig'
make[5]: *** [Makefile:459: all-recursive] Error 1
make[5]: Leaving directory '/home/wlodek/kea/premium/src/hooks/d2'
make[4]: *** [Makefile:456: all-recursive] Error 1
make[4]: Leaving directory '/home/wlodek/kea/premium/src/hooks'
make[3]: *** [Makefile:456: all-recursive] Error 1
make[3]: Leaving directory '/home/wlodek/kea/premium/src'
make[2]: *** [Makefile:457: all-recursive] Error 1
make[2]: Leaving directory '/home/wlodek/kea/premium'
make[1]: *** [Makefile:626: all-recursive] Error 1
make[1]: Leaving directory '/home/wlodek/kea'
make: *** [Makefile:514: all] Error 2