Makefile: reduce clutter if they haven't run ./configure

It's hard to see the error:

Before:

    $ make
    File config.vars not found: you must run ./configure before running make.
    CC: cc -DBINTOPKGLIBEXECDIR="../libexec/c-lightning"    -I ccan -I external/libwally-core/include/ -I external/libwally-core/src/secp256k1/include/ -I external/jsmn/ -I external/libbacktrace/ -I external/x86_64-linux-gnu/libbacktrace-build -I external/libsodium/src/libsodium/include -I external/libsodium/src/libsodium/include/sodium -I external/x86_64-linux-gnu/libsodium-build/src/libsodium/include -I . -I/usr/local/include    -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS   -DBUILD_ELEMENTS=1 -c -o
    LD: cc  -fsanitize=address     -Lexternal/x86_64-linux-gnu -lwallycore -lsecp256k1 -ljsmn -lbacktrace -lsodium -L/usr/local/lib -lm -lgmp -lsqlite3 -lz  -o

After:

    $ make
    File config.vars not found: you must run ./configure before running make.
    make: *** No rule to make target 'config.vars', needed by 'show-flags'.  Stop.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -233,7 +233,7 @@ endif
 
 default: show-flags all-programs all-test-programs doc-all
 
-show-flags:
+show-flags: config.vars
 	@$(ECHO) "CC: $(CC) $(CFLAGS) -c -o"
 	@$(ECHO) "LD: $(LINK.o) $(filter-out %.a,$^) $(LOADLIBES) $(EXTERNAL_LDLIBS) $(LDLIBS) -o"