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6.2.1 Recursively Expanded Variable Assignment
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The first flavor of variable is a "recursively expanded" variable.
Variables of this sort are defined by lines using '=' (*note Setting
Variables: Setting.) or by the 'define' directive (*note Defining
Multi-Line Variables: Multi-Line.).  The value you specify is installed
verbatim; if it contains references to other variables, these references
are expanded whenever this variable is substituted (in the course of
expanding some other string).  When this happens, it is called
"recursive expansion".

   For example,

     foo = $(bar)
     bar = $(ugh)
     ugh = Huh?

     all:;echo $(foo)

will echo 'Huh?': '$(foo)' expands to '$(bar)' which expands to '$(ugh)'
which finally expands to 'Huh?'.

   This flavor of variable is the only sort supported by most other
versions of 'make'.  It has its advantages and its disadvantages.  An
advantage (most would say) is that:

     CFLAGS = $(include_dirs) -O
     include_dirs = -Ifoo -Ibar

will do what was intended: when 'CFLAGS' is expanded in a recipe, it
will expand to '-Ifoo -Ibar -O'.  A major disadvantage is that you
cannot append something on the end of a variable, as in

     CFLAGS = $(CFLAGS) -O

because it will cause an infinite loop in the variable expansion.
(Actually 'make' detects the infinite loop and reports an error.)

   Another disadvantage is that any functions (*note Functions for
Transforming Text: Functions.) referenced in the definition will be
executed every time the variable is expanded.  This makes 'make' run
slower; worse, it causes the 'wildcard' and 'shell' functions to give
unpredictable results because you cannot easily control when they are
called, or even how many times.