[automake] How to write multiple conditions in Makefile.am with "else if"

ifdef $(HAVE_CLIENT)
libtest_LIBS = \
    $(top_builddir)/libclient.la
else
ifdef $(HAVE_SERVER)
libtest_LIBS = \
    $(top_builddir)/libserver.la
else
libtest_LIBS = 
endif
endif

NOTE: DO NOT indent the if then it don't work!