GetBooleanExpr
writable = GetBooleanExpr("${MAKE_OBJDIR_CHECK_WRITABLE}", true);
!GetBooleanExpr("${.MAKE.DIE_QUIETLY}", true))
if (GetBooleanExpr("${MAKE_DEBUG_OBJDIR_CHECK_WRITABLE}",
expandVars = GetBooleanExpr("${.MAKE.EXPAND_VARIABLES}",
bool GetBooleanExpr(const char *, bool);
bool targetVarsEnabled = GetBooleanExpr(