Remove -Wshadow -Wjump-misses-init -Wundef

Doing this now specifically because clang does not have
-Wjump-misses-init, but I've been looking for an excuse to remove these
for a while.

These warning flags create more annoyance than they add value. There is
probably a reason they aren't included in -Wall + -Wextra.

-Wshadow specifically is potentially harmful as it forces coming up with
new, sometimes less descriptive names for repeated variables.

Dependent projects should use different flags for their dependencies if
this introduces problems.

Makefile

@@ -41,8 +41,7 @@ override CFLAGS += -DLFS_YES_TRACE
 endif
 override CFLAGS += -g3
 override CFLAGS += -I.
-override CFLAGS += -std=c99 -Wall -pedantic
+override CFLAGS += -std=c99 -Wall -Wextra -pedantic
-override CFLAGS += -Wextra -Wshadow -Wjump-misses-init -Wundef
 
 ifdef VERBOSE
 override TESTFLAGS     += -v