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(make.info)Conditional Example

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Example of a Conditional
========================

   The following example of a conditional tells `make' to use one set
of libraries if the `CC' variable is `gcc', and a different set of
libraries otherwise.  It works by controlling which of two command
lines will be used as the command for a rule.  The result is that
`CC=gcc' as an argument to `make' changes not only which compiler is
used but also which libraries are linked.

     libs_for_gcc = -lgnu
     normal_libs =
     
     foo: $(objects)
     ifeq ($(CC),gcc)
             $(CC) -o foo $(objects) $(libs_for_gcc)
     else
             $(CC) -o foo $(objects) $(normal_libs)
     endif

   This conditional uses three directives: one `ifeq', one `else' and
one `endif'.

   The `ifeq' directive begins the conditional, and specifies the
condition.  It contains two arguments, separated by a comma and
surrounded by parentheses.  Variable substitution is performed on both
arguments and then they are compared.  The lines of the makefile
following the `ifeq' are obeyed if the two arguments match; otherwise
they are ignored.

   The `else' directive causes the following lines to be obeyed if the
previous conditional failed.  In the example above, this means that the
second alternative linking command is used whenever the first
alternative is not used.  It is optional to have an `else' in a
conditional.

   The `endif' directive ends the conditional.  Every conditional must
end with an `endif'.  Unconditional makefile text follows.

   As this example illustrates, conditionals work at the textual level:
the lines of the conditional are treated as part of the makefile, or
ignored, according to the condition.  This is why the larger syntactic
units of the makefile, such as rules, may cross the beginning or the
end of the conditional.

   When the variable `CC' has the value `gcc', the above example has
this effect:

     foo: $(objects)
             $(CC) -o foo $(objects) $(libs_for_gcc)

When the variable `CC' has any other value, the effect is this:

     foo: $(objects)
             $(CC) -o foo $(objects) $(normal_libs)

   Equivalent results can be obtained in another way by
conditionalizing a variable assignment and then using the variable
unconditionally:

     libs_for_gcc = -lgnu
     normal_libs =
     
     ifeq ($(CC),gcc)
       libs=$(libs_for_gcc)
     else
       libs=$(normal_libs)
     endif
     
     foo: $(objects)
             $(CC) -o foo $(objects) $(libs)