Re-allow disabling vmalloc for release builds (re: 399886da)

Well, that commit was based on a silly oversight: of course it's
necessary to pass ${KSH_RELFLAGS} to the feature tests too as they
use this flag to determine whether to enable or disable vmalloc.

On further analysis I think the annoying warnings can be solved in
a different way. Quotes (single or double) in 'exec -' commands
don't seem to be special to mamake at all; it looks like they are
passed on to the shell as is. So Mamfile variables are expanded and
the expansions backslash-escaped the same way regardless of quotes.
Which means we can make the shell remove the unwanted level of
backslashes by using double instead of single quotes.

src/*/*/Mamfile:
- On iffe commands, restore ${KSH_RELFLAGS}, using double quotes to
  group the compiler command as one argument to iffe.

src/cmd/builtin/Mamfile

@@ -66,7 +66,7 @@ make install
 					meta FEATURE/pty features/%>FEATURE/% features/pty pty
 					make features/pty
 					done features/pty
-					exec - iffe ${IFFEFLAGS} -v -c '${CC} ${mam_cc_FLAGS} ${CCFLAGS} ${LDFLAGS} -lm' ref ${mam_cc_L+-L${INSTALLROOT}/lib} -I${PACKAGE_ast_INCLUDE} -I${INSTALLROOT}/include ${mam_libast} ${mam_libcmd} : run features/pty
+					exec - iffe ${IFFEFLAGS} -v -c "${CC} ${mam_cc_FLAGS} ${KSH_RELFLAGS} ${CCFLAGS} ${LDFLAGS} -lm" ref ${mam_cc_L+-L${INSTALLROOT}/lib} -I${PACKAGE_ast_INCLUDE} -I${INSTALLROOT}/include ${mam_libast} ${mam_libcmd} : run features/pty
 				done FEATURE/pty generated
 				make ${PACKAGE_ast_INCLUDE}/ast_time.h implicit
 				done ${PACKAGE_ast_INCLUDE}/ast_time.h