Import changes from IMath versions (1.3, 1.29].

Upstream fixed bugs over the years, but none are confirmed to have
affected pgcrypto.  We're better off naively tracking upstream than
reactively maintaining a twelve-year-old snapshot of upstream.  Add a
header comment describing the synchronization procedure.  Discard use of
INVERT_COMPARE_RESULT(); the domain of the comparisons in question is
{-1,0,1}, controlled entirely by code in imath.c.

Andrew Gierth reviewed the Makefile change.  Tom Lane reviewed the
synchronization procedure description.

Discussion: https://postgr.es/m/20190203035704.GA6226@rfd.leadboat.com

contrib/pgcrypto/Makefile

@@ -59,6 +59,9 @@ SHLIB_LINK += $(filter -leay32, $(LIBS))
 SHLIB_LINK += -lws2_32
 endif
 
+# Upstream uses a larger subset of C99.
+imath.o: CFLAGS+=$(PERMIT_DECLARATION_AFTER_STATEMENT)
+
 rijndael.o: rijndael.tbl
 
 rijndael.tbl: