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.

SCons is an Open Source software construction tool—that is, a next-generation build tool. Think of SCons as an improved, cross-platform substitute for the classic Make utility with integrated functionality similar to autoconf/automake and compiler caches such as ccache. In short, SCons is an easier, more reliable and faster way to build software.
SCons