( Eval Function

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 8.8 The `eval' Function
 The `eval' function is very special: it allows you to define new
 makefile constructs that are not constant; which are the result of
 evaluating other variables and functions.  The argument to the `eval'
 function is expanded, then the results of that expansion are parsed as
 makefile syntax.  The expanded results can define new `make' variables,
 targets, implicit or explicit rules, etc.
    The result of the `eval' function is always the empty string; thus,
 it can be placed virtually anywhere in a makefile without causing
 syntax errors.
    It's important to realize that the `eval' argument is expanded
 _twice_; first by the `eval' function, then the results of that
 expansion are expanded again when they are parsed as makefile syntax.
 This means you may need to provide extra levels of escaping for "$"
 characters when using `eval'.  The `value' function ( Value
 Function) can sometimes be useful in these situations, to circumvent
 unwanted expansions.
    Here is an example of how `eval' can be used; this example combines
 a number of concepts and other functions.  Although it might seem
 overly complex to use `eval' in this example, rather than just writing
 out the rules, consider two things: first, the template definition (in
 `PROGRAM_template') could need to be much more complex than it is here;
 and second, you might put the complex, "generic" part of this example
 into another makefile, then include it in all the individual makefiles.
 Now your individual makefiles are quite straightforward.
      PROGRAMS    = server client
      server_OBJS = server.o server_priv.o server_access.o
      server_LIBS = priv protocol
      client_OBJS = client.o client_api.o client_mem.o
      client_LIBS = protocol
      # Everything after this is generic
      .PHONY: all
      all: $(PROGRAMS)
      define PROGRAM_template
       $(1): $$($(1)_OBJS) $$($(1)_LIBS:%=-l%)
       ALL_OBJS   += $$($(1)_OBJS)
      $(foreach prog,$(PROGRAMS),$(eval $(call PROGRAM_template,$(prog))))
              $(LINK.o) $^ $(LDLIBS) -o $@
              rm -f $(ALL_OBJS) $(PROGRAMS)
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