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if Function
The if function provides support for conditional expansion in a
functional context (as opposed to the GNU make makefile
conditionals such as ifeq (see section Syntax of Conditionals).
An if function call can contain either two or three arguments:
$(if condition,then-part[,else-part])
The first argument, condition, first has all preceding and trailing whitespace stripped, then is expanded. If it expands to any non-empty string, then the condition is considered to be true. If it expands to an empty string, the condition is considered to be false.
If the condition is true then the second argument, then-part, is
evaluated and this is used as the result of the evaluation of the entire
if function.
If the condition is false then the third argument, else-part, is
evaluated and this is the result of the if function. If there is
no third argument, the if function evaluates to nothing (the
empty string).
Note that only one of the then-part or the else-part will be
evaluated, never both. Thus, either can contain side-effects (such as
shell function calls, etc.)
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