--disable-jit-support should disable YJIT successfully

Even if `rustc` is available, it should not be an error unless
`--enable-yjit` is explicitly given.

common.mk

@@ -303,7 +303,7 @@ showflags:
 	"	LC_ALL = $(LC_ALL)" \
 	"	LC_CTYPE = $(LC_CTYPE)" \
 	"	MFLAGS = $(MFLAGS)" \
-	"	RUST = $(RUST)" \
+	"	RUSTC = $(RUSTC)" \
 	"	YJIT_RUSTC_ARGS = $(YJIT_RUSTC_ARGS)" \
 	$(MESSAGE_END)
 	-@$(CC_VERSION)

configure.ac

@@ -3754,15 +3754,19 @@ AC_ARG_ENABLE(yjit,
     AS_HELP_STRING([--enable-yjit],
     [enable experimental in-process JIT compiler that requires Rust build tools [default=no]]),
     [YJIT_SUPPORT=$enableval],
-    [
-        AS_IF([test x"$RUSTC" != "xno"],
+    [AS_CASE(["$enable_jit_support:$YJIT_TARGET_OK:$RUSTC"],
+        [no:*|yes:no:*|yes:yes:no], [
+            YJIT_SUPPORT=no
+        ],
+        [yes:yes:*], [
             AS_IF([ echo "fn main() { let x = 1; format!(\"{x}\"); }" | $RUSTC - --emit asm=/dev/null ],
-                [YJIT_SUPPORT="$YJIT_TARGET_OK"],
+                [YJIT_SUPPORT=yes],
                 [YJIT_SUPPORT=no]
-            ),
+            )
+        ], [
             [YJIT_SUPPORT=no]
-        )
-    ]
+        ]
+    )]
 )
 
 CARGO=