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The following snippet of code can be used to check for the existence of a file from within a Makefile.

ifneq ("$(wildcard $(PATH_TO_FILE))","")
FILE_EXISTS = 1
else
FILE_EXISTS = 0
endif

Quoting from make documentation.

$(wildcard pattern)
The argument pattern is a file name pattern, typically containing
wildcard characters (as in shell file name patterns). The result of
wildcard is a space-separated list of the names of existing files
that match the pattern.

In this present case, we are not using any wildcards, but the absolute path to the file.