Disable sign compare on illumos

On illumos, the UINT8_MAX is defined as 255U, see:
https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/sys/int_limits.h#L92

Makefile

@@ -17,7 +17,7 @@ else
     PG_CPPFLAGS = -std=c99 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-implicit-fallthrough -Iinclude -I$(libpq_srcdir)
 endif
 ifeq ($(shell uname -s),SunOS)
-    PG_CPPFLAGS += -D__EXTENSIONS__
+    PG_CPPFLAGS += -Wno-sign-compare -D__EXTENSIONS__
 endif
 SHLIB_LINK = $(libpq)
 EXTRA_CLEAN += $(addprefix src/,*.gcno *.gcda) # clean up after profiling runs