Another way to set the value of a variable is to use the define
directive. This directive has an unusual syntax which allows newline
characters to be included in the value, which is convenient for defining
both canned sequences of commands
(see Defining Canned Command Sequences), and also
sections of makefile syntax to use with eval (see Eval Function).
The define directive is followed on the same line by the name of the
variable and nothing more. The value to give the variable appears on the
following lines. The end of the value is marked by a line containing just
the word endef. Aside from this difference in syntax, define
works just like ‘=’: it creates a recursively-expanded variable
(see The Two Flavors of Variables).
The variable name may contain function and variable references, which
are expanded when the directive is read to find the actual variable name
to use.
You may nest define directives: make will keep track of
nested directives and report an error if they are not all properly
closed with endef. Note that lines beginning with tab
characters are considered part of a command script, so any
define or endef strings appearing on such a line will
not be considered make operators.
define two-lines
echo foo
echo $(bar)
endef
The value in an ordinary assignment cannot contain a newline; but the
newlines that separate the lines of the value in a define become
part of the variable's value (except for the final newline which precedes
the endef and is not considered part of the value).
When used in a command script, the previous example is functionally equivalent to this:
two-lines = echo foo; echo $(bar)
since two commands separated by semicolon behave much like two separate
shell commands. However, note that using two separate lines means
make will invoke the shell twice, running an independent subshell
for each line. See Command Execution.
If you want variable definitions made with define to take
precedence over command-line variable definitions, you can use the
override directive together with define:
override define two-lines
foo
$(bar)
endef