Use of ":=" in src/Makefile.am is unportable (bug 14420)

Use of ":=" in src/Makefile.am is unportable (bug 14420)

Building 2.5.91 on Solaris with the native make(1) yields

...
Making all in src
make: Fatal error in reader: Makefile, line 313: Unexpected end of line seen
Current working directory /tmp/fontconfig-2.5.91/src
*** Error code 1

This is due to the following line (src/Makefile.am:143):

    CLEANFILES := $(ALIAS_FILES)

Changing that to a standard assignment ("=") fixes the problem.

I believe the ":=" is a typo. ALIAS_FILES is just a statically assigned
variable; it's not like evaluating it more than once would be a problem.