allow setting the standard

CSTD can be manually set near the top of src/Makefile, util/Makefile
or by including c99=1 or C99=1 on the build make command line if
using the linux.370 or macOS.370 hints files.

sys/unix/hints/include/compiler.370

@@ -149,8 +149,18 @@ endif  # CPLUSPLUS_NEED17
 endif  # end of clang++-specific section
 CXX=$(CCXX)
 endif  # CPLUSPLUS_NEEDED
-
 endif   # MAKEFILE_SRC
 
+ifeq "$(c99)" "1"
+WANT_C99=1
+endif
+ifeq "$(C99)" "1"
+WANT_C99=1
+endif
+
+ifeq "$(WANT_C99)" "1"
+CSTD = -std=c99
+endif
+
 #end of compiler.370
 #------------------------------------------------------------------------------