Conditional judgment
The syntax of the conditional expression is:
<conditional-directive>
<text-if-true>
endif
以及：
<conditional-directive>
<text-if-true>
else
<text-if-false>
endif
Which <conditional-directive>represents conditional keywords, such as "ifeq". There are four keywords, as follows:
1.“ifeq”
ifeq (<arg1>, <arg2> )
ifeq '<arg1>' '<arg2>'
ifeq "<arg1>" "<arg2>"
ifeq "<arg1>" '<arg2>'
ifeq '<arg1>' "<arg2>"
Compare whether the values of the parameters "arg1" and "arg2" are the same, and if they are the same, it is true.
2. The second condition keyword is "ifneq". The syntax is:
ifneq (<arg1>, <arg2> )
ifneq '<arg1>' '<arg2>'
ifneq "<arg1>" "<arg2>"
ifneq "<arg1>" '<arg2>'
ifneq '<arg1>' "<arg2>"
It compares whether the values of the parameters "arg1" and "arg2" are the same, and if they are different, it is true.
3. The third condition keyword is "ifdef". The syntax is:
ifdef <variable-name>
If <variable-name>the value of the variable is not empty, the expression is true. Otherwise, the expression is false.
4. The fourth condition keyword is "ifndef". The syntax is:
ifndef <variable-name>
If the <variable-name>value is empty, the expression is true.