makefile ifndef

ifndef is a directive in Makefile used to check if a variable or macro is defined. It is followed by the name of the variable or macro that we want to check. If the variable or macro is not defined, then the code block following ifndef will be executed. If it is defined, the code block will be skipped.

For example, consider the following Makefile snippet:

ifndef VARIABLE_NAME
    # Code block to be executed if VARIABLE_NAME is not defined
else
    # Code block to be executed if VARIABLE_NAME is defined
endif

In this example, if the variable `VARIABLE_NAME` is not defined, the code block following `ifndef` will be executed. Otherwise, the code block following `else` will be executed.

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