Add support for build statement implicit outputs

Some build rules produce outputs that are not mentioned on the command
line but that should be part of the build graph.  Such outputs should
not be named in the `$out` variable.  Extend the build statement syntax
to support specification of implicit outputs using the syntax
`| out1 out2` after the explicit outputs and before the `:`.

For example, compilation of a Fortran source file `foo.f90` that defines
`MODULE FOO` may now be specified as:

    rule fc
      command = f95 -c $in -o $out
    build foo.o | foo.mod: fc foo.f90

The `foo.mod` file is an implicit output generated by the compiler based
on the content of the source file and not mentioned on the command line.

1 files changed, 26 insertions, 1 deletions

diff --git a/doc/manual.asciidoc b/doc/manual.asciidoc
index ab5c945..4e73df3 100644
--- a/doc/manual.asciidoc
+++ b/doc/manual.asciidoc
@@ -689,6 +689,10 @@ A file is a series of declarations.  A declaration can be one of:
    Order-only dependencies may be tacked on the end with +||
    _dependency1_ _dependency2_+.  (See <<ref_dependencies,the reference on
    dependency types>>.)
++
+Implicit outputs _(available since Ninja 1.7)_ may be added before
+the `:` with +| _output1_ _output2_+ and do not appear in `$out`.
+(See <<ref_outputs,the reference on output types>>.)
 
 3. Variable declarations, which look like +_variable_ = _value_+.
 
@@ -872,6 +876,27 @@ interpretation of the command (such as the use of `&&` to chain
 multiple commands), make the command execute the Windows shell by
 prefixing the command with `cmd /c`.
 
+[[ref_outputs]]
+Build outputs
+~~~~~~~~~~~~~
+
+There are two types of build outputs which are subtly different.
+
+1. _Explicit outputs_, as listed in a build line.  These are
+   available as the `$out` variable in the rule.
++
+This is the standard form of output to be used for e.g. the
+object file of a compile command.
+
+2. _Implicit outputs_, as listed in a build line with the syntax +|
+   _out1_ _out2_+ + before the `:` of a build line _(available since
+   Ninja 1.7)_.  The semantics are identical to explicit outputs,
+  the only difference is that implicit outputs don't show up in the
+  `$out` variable.
++
+This is for expressing outputs that don't show up on the
+command line of the command.
+
 [[ref_dependencies]]
 Build dependencies
 ~~~~~~~~~~~~~~~~~~
@@ -883,7 +908,7 @@ There are three types of build dependencies which are subtly different.
    cause the output to be rebuilt; if these file are missing and
    Ninja doesn't know how to build them, the build is aborted.
 +
-This is the standard form of dependency to be used for e.g. the
+This is the standard form of dependency to be used e.g. for the
 source file of a compile command.
 
 2. _Implicit dependencies_, either as picked up from