TF/02 Bugzilla 139: Avoid using the += operator in the modular lookup
makefiles for portability to HP-UX and POSIX correctness.
+PP/01 Permit LOOKUP_foo enabling on the make command-line.
+ Also via indented variable definition in the Makefile.
+ (Debugging by Oliver Heesakkers).
+
Exim version 4.74
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