try_compile: Add policy CMP0066 to honor CMAKE_<LANG>_FLAGS_<CONFIG>

In the `try_compile` source file signature we propagate the caller's
value of `CMAKE_<LANG>_FLAGS` into the test project.  Extend this to
propagate `CMAKE_<LANG>_FLAGS_<CONFIG>` too instead of always using the
default value in the test project.  This will be useful, for example, to
allow the MSVC runtime library to be changed (e.g. `-MDd` => `-MTd`).
However, some projects may currently depend on this not being done,
so we need to activate the behavior using a policy.

This change was originally made by commit v3.6.0-rc1~160^2 (try_compile:
Honor CMAKE_<LANG>_FLAGS_<CONFIG> changes, 2016-04-11) but without the
policy and so had to be reverted during the 3.6 release candidate cycle.

Fixes #16174.

4 files changed, 44 insertions, 0 deletions

diff --git a/Help/manual/cmake-policies.7.rst b/Help/manual/cmake-policies.7.rst
index 43f4637..0cfe983 100644
--- a/Help/manual/cmake-policies.7.rst
+++ b/Help/manual/cmake-policies.7.rst
@@ -51,6 +51,14 @@ The :variable:`CMAKE_MINIMUM_REQUIRED_VERSION` variable may also be used
 to determine whether to report an error on use of deprecated macros or
 functions.
 
+Policies Introduced by CMake 3.7
+================================
+
+.. toctree::
+   :maxdepth: 1
+
+   CMP0066: Honor per-config flags in try_compile() source-file signature. </policy/CMP0066>
+
 Policies Introduced by CMake 3.4
 ================================
 
diff --git a/Help/policy/CMP0066.rst b/Help/policy/CMP0066.rst
new file mode 100644
index 0000000..d1dcb0e
--- /dev/null
+++ b/Help/policy/CMP0066.rst
@@ -0,0 +1,27 @@
+CMP0066
+-------
+
+Honor per-config flags in :command:`try_compile` source-file signature.
+
+The source file signature of the :command:`try_compile` command uses the value
+of the :variable:`CMAKE_<LANG>_FLAGS` variable in the test project so that the
+test compilation works as it would in the main project.  However, CMake 3.6 and
+below do not also honor config-specific compiler flags such as those in the
+:variable:`CMAKE_<LANG>_FLAGS_DEBUG` variable.  CMake 3.7 and above prefer to
+honor config-specific compiler flags too.  This policy provides compatibility
+for projects that do not expect config-specific compiler flags to be used.
+
+The ``OLD`` behavior of this policy is to ignore config-specific flag
+variables like :variable:`CMAKE_<LANG>_FLAGS_DEBUG` and only use CMake's
+built-in defaults for the current compiler and platform.
+
+The ``NEW`` behavior of this policy is to honor config-specific flag
+variabldes like :variable:`CMAKE_<LANG>_FLAGS_DEBUG`.
+
+This policy was introduced in CMake version 3.7.  Unlike most policies,
+CMake version |release| does *not* warn by default when this policy
+is not set and simply uses OLD behavior.  See documentation of the
+:variable:`CMAKE_POLICY_WARNING_CMP0066 <CMAKE_POLICY_WARNING_CMP<NNNN>>`
+variable to control the warning.
+
+.. include:: DEPRECATED.txt
diff --git a/Help/release/dev/try_compile-config-flags.rst b/Help/release/dev/try_compile-config-flags.rst
new file mode 100644
index 0000000..ebfd6a4
--- /dev/null
+++ b/Help/release/dev/try_compile-config-flags.rst
@@ -0,0 +1,7 @@
+try_compile-config-flags
+------------------------
+
+* The :command:`try_compile` command source file signature now honors
+  configuration-specific flags (e.g. :variable:`CMAKE_<LANG>_FLAGS_DEBUG`)
+  in the generated test project.  Previously only the default such flags
+  for the current toolchain were used.  See policy :policy:`CMP0066`.
diff --git a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
index 582f9e4..36cf75f 100644
--- a/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
+++ b/Help/variable/CMAKE_POLICY_WARNING_CMPNNNN.rst
@@ -15,6 +15,8 @@ warn by default:
   policy :policy:`CMP0060`.
 * ``CMAKE_POLICY_WARNING_CMP0065`` controls the warning for
   policy :policy:`CMP0065`.
+* ``CMAKE_POLICY_WARNING_CMP0066`` controls the warning for
+  policy :policy:`CMP0066`.
 
 This variable should not be set by a project in CMake code.  Project
 developers running CMake may set this variable in their cache to