bison-3.1.90 does not build on linux

[Denis Excoffier]

Hello,

bison-3.1.90 does not build on (my) linux because (at least) clock_gettime() is 
not found in any library given.

It seems that LIB_CLOCK_GETTIME and LIB_GETHRXTIME are both correctly assigned 
to -lrt but nobody uses these macros.

Using (in Makefile.in):
src_bison_LDADD = $(LDADD) $(LIB_CLOCK_GETTIME) $(LIB_GETHRXTIME)

clears the problem.

Regards,

Denis Excoffier.

P.S. Seen the new 3.1.91 but not tested. Seems (by inspection and diff'ing) 
that the problem is still here though.