Force --enable-debug on FreeBSD if INVARIANTS is set

There's already logic to force INVARIANTS on for building if it's
present in the running kernel; however, not having DEBUG enabled
when DEBUG and INVARIANTS are can cause strange panics.

Closes: openzfs#12163

Signed-off-by: Rich Ercolani <rincebrain@gmail.com>

config/zfs-build.m4

@@ -34,6 +34,9 @@ dnl # When debugging is enabled:
 dnl # - Enable all ASSERTs (-DDEBUG)
 dnl # - Promote all compiler warnings to errors (-Werror)
 dnl #
+dnl # (If INVARIANTS is detected, we need to force DEBUG, or strange panics
+dnl # can ensue.)
+dnl #
 AC_DEFUN([ZFS_AC_DEBUG], [
 	AC_MSG_CHECKING([whether assertion support will be enabled])
 	AC_ARG_ENABLE([debug],
@@ -49,6 +52,20 @@ AC_DEFUN([ZFS_AC_DEBUG], [
 		[ZFS_AC_DEBUG_DISABLE],
 		[AC_MSG_ERROR([Unknown option $enable_debug])])
 
+	AS_CASE(["x$enable_invariants"],
+		["xyes"],
+		[],
+		["xno"],
+		[],
+		[ZFS_AC_DEBUG_INVARIANTS_DETECT])
+
+	AS_CASE(["x$enable_invariants"],
+		["xyes"],
+		[ZFS_AC_DEBUG_ENABLE],
+		["xno"],
+		[],
+		[AC_MSG_ERROR([Unknown option $enable_invariants])])
+
 	AC_SUBST(DEBUG_CFLAGS)
 	AC_SUBST(DEBUG_CPPFLAGS)
 	AC_SUBST(DEBUG_LDFLAGS)