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.
patternis a file name pattern, typically containing
wildcard characters (as in shell file name patterns). The result of
wildcardis 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.