Make

If file1 does not exist then $(wildcard file1) will evaluate to an empty string. ifeq ($(wildcard file1),) CLEAN_SRC = else CLEAN_SRC = *.h file3 endif
 * http://www.gnu.org/software/make/manual/make.html
 * http://www.apl.jhu.edu/Misc/Unix-info/make/make_6.html
 * http://www.freebsd.org/doc/en/books/porters-handbook/makefile-options.html
 * http://www.weask.us/entry/conditional-set-makefile-variable-testing-file-exists


 * http://www.gnu.org/software/make/manual/make.html#Call-Function