I would accept ldav1s' answer if I were you, but I just want to point out that 'else if' can be written in terms of 'else's and 'if's in any language:
if HAVE_CLIENT
libtest_LIBS = $(top_builddir)/libclient.la
else
if HAVE_SERVER
libtest_LIBS = $(top_builddir)/libserver.la
else
libtest_LIBS =
endif
endif
(The indentation is for clarity. Don't indent the lines, they won't work.)