Make --enable-debug fail when given bogus arguments

Currently, bogus options to --enable-debug become --disable-debug. That
means that passing --enable-debug=true is analogous to --disable-debug,
but the result is counterintuitive. We switch to AS_CASE to allow us to
fail when given a bogus option.

Also, we modify the text printed to clarify that --enable-debug enables
assertions.

Signed-off-by: Richard Yao <richard.yao@clusterhq.com>

config/zfs-build.m4

@@ -6,30 +6,37 @@ AC_DEFUN([ZFS_AC_LICENSE], [
 	AC_MSG_RESULT([$ZFS_META_LICENSE])
 ])
 
+AC_DEFUN([ZFS_AC_DEBUG_ENABLE], [
+	KERNELCPPFLAGS="${KERNELCPPFLAGS} -DDEBUG -Werror"
+	HOSTCFLAGS="${HOSTCFLAGS} -DDEBUG -Werror"
+	DEBUG_CFLAGS="-DDEBUG -Werror"
+	DEBUG_STACKFLAGS="-fstack-check"
+	DEBUG_ZFS="_with_debug"
+	AC_DEFINE(ZFS_DEBUG, 1, [zfs debugging enabled])
+])
+
+AC_DEFUN([ZFS_AC_DEBUG_DISABLE], [
+	KERNELCPPFLAGS="${KERNELCPPFLAGS} -DNDEBUG "
+	HOSTCFLAGS="${HOSTCFLAGS} -DNDEBUG "
+	DEBUG_CFLAGS="-DNDEBUG"
+	DEBUG_STACKFLAGS=""
+	DEBUG_ZFS="_without_debug"
+])
+
 AC_DEFUN([ZFS_AC_DEBUG], [
-	AC_MSG_CHECKING([whether debugging is enabled])
+	AC_MSG_CHECKING([whether assertion support will be enabled])
 	AC_ARG_ENABLE([debug],
 		[AS_HELP_STRING([--enable-debug],
-		[Enable generic debug support @<:@default=no@:>@])],
+		[Enable assertion support @<:@default=no@:>@])],
 		[],
 		[enable_debug=no])
 
-	AS_IF([test "x$enable_debug" = xyes],
-	[
-		KERNELCPPFLAGS="${KERNELCPPFLAGS} -DDEBUG -Werror"
-		HOSTCFLAGS="${HOSTCFLAGS} -DDEBUG -Werror"
-		DEBUG_CFLAGS="-DDEBUG -Werror"
-		DEBUG_STACKFLAGS="-fstack-check"
-		DEBUG_ZFS="_with_debug"
-		AC_DEFINE(ZFS_DEBUG, 1, [zfs debugging enabled])
-	],
-	[
-		KERNELCPPFLAGS="${KERNELCPPFLAGS} -DNDEBUG "
-		HOSTCFLAGS="${HOSTCFLAGS} -DNDEBUG "
-		DEBUG_CFLAGS="-DNDEBUG"
-		DEBUG_STACKFLAGS=""
-		DEBUG_ZFS="_without_debug"
-	])
+	AS_CASE(["x$enable_debug"],
+		["xyes"],
+		[ZFS_AC_DEBUG_ENABLE],
+		["xno"],
+		[ZFS_AC_DEBUG_DISABLE],
+		[AC_MSG_ERROR([Unknown option $enable_debug])])
 
 	AC_SUBST(DEBUG_CFLAGS)
 	AC_SUBST(DEBUG_STACKFLAGS)