GNU make manual 翻译( 一百八十)

7.2 Syntax of Conditionals
==========================

The syntax of a simple conditional with no `else' is as follows:


     CONDITIONAL-DIRECTIVE
     TEXT-IF-TRUE
     endif

The TEXT-IF-TRUE may be any lines of text, to be considered as part of
the makefile if the condition is true.  If the condition is false, no
text is used instead.

   The syntax of a complex conditional is as follows:

     CONDITIONAL-DIRECTIVE
     TEXT-IF-TRUE
     else
     TEXT-IF-FALSE
     endif

   or:

     CONDITIONAL-DIRECTIVE
     TEXT-IF-ONE-IS-TRUE
     else CONDITIONAL-DIRECTIVE
     TEXT-IF-TRUE
     else
     TEXT-IF-FALSE
     endif

There can be as many "`else' CONDITIONAL-DIRECTIVE" clauses as
necessary.  Once a given condition is true, TEXT-IF-TRUE is used and no
other clause is used; if no condition is true then TEXT-IF-FALSE is
used.  The TEXT-IF-TRUE and TEXT-IF-FALSE can be any number of lines of
text.