Ninja: Add policy to require explicit custom command byproducts

Add policy CMP0058 to avoid generating 'phony' ninja rules for unknown
custom command dependencies.  This requires projects to specify their
custom command byproducts explicitly.  With this requirement we no
longer have to assume that unknown custom command dependencies are
generated and can instead simply assume they are source files expected
to exist when the build starts.  This is particularly important in
in-source builds.  It is also helpful for out-of-source builds to allow
Ninja to diagnose missing files before running custom command rules that
depend on them.

1 files changed, 19 insertions, 0 deletions

diff --git a/Tests/RunCMake/Ninja/CMP0058-WARN-no-stderr.txt b/Tests/RunCMake/Ninja/CMP0058-WARN-no-stderr.txt
new file mode 100644
index 0000000..439a2d9
--- /dev/null
+++ b/Tests/RunCMake/Ninja/CMP0058-WARN-no-stderr.txt
@@ -0,0 +1,19 @@
+^CMake Warning \(dev\):
+  Policy CMP0058 is not set: Ninja requires custom command byproducts to be
+  explicit.  Run "cmake --help-policy CMP0058" for policy details.  Use the
+  cmake_policy command to set the policy and suppress this warning.
+
+  This project specifies custom command DEPENDS on files in the build tree
+  that are not specified as the OUTPUT or BYPRODUCTS of any
+  add_custom_command or add_custom_target:
+
+   byproduct1a
+   byproduct1b
+
+  For compatibility with versions of CMake that did not have the BYPRODUCTS
+  option, CMake is generating phony rules for such files to convince 'ninja'
+  to build.
+
+  Project authors should add the missing BYPRODUCTS or OUTPUT options to the
+  custom commands that produce these files.
+This warning is for project developers.  Use -Wno-dev to suppress it.