find_package: Add variable to make package REQUIRED

Add a `CMAKE_REQUIRE_FIND_PACKAGE_<PackageName>` variable is complement
to `CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` with just the opposite
behaviour: it turns non-required find_package call into the required one.

While optional package dependencies usually result in simple and clean
build logic, sometimes people want to be sure those optional
dependencies will be found and used. Examples are reproducible builds
and build instructions for 3rd parties. People choose to make
find_package calls REQUIRED and put them behind an option(). Such
workarounds blend build logic with build environment management and do
not look elegant.

1 files changed, 9 insertions, 2 deletions

diff --git a/Help/command/find_package.rst b/Help/command/find_package.rst
index 3dfd62f..7febd5d 100644
--- a/Help/command/find_package.rst
+++ b/Help/command/find_package.rst
@@ -448,8 +448,15 @@ which the file is found.  The :variable:`CMAKE_FIND_PACKAGE_RESOLVE_SYMLINKS`
 variable may be set to ``TRUE`` before calling ``find_package`` in order
 to resolve symbolic links and store the real path to the file.
 
-Every non-REQUIRED ``find_package`` call can be disabled by setting the
-:variable:`CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` variable to ``TRUE``.
+Every non-REQUIRED ``find_package`` call can be disabled or made REQUIRED:
+
+* Setting the :variable:`CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` variable
+  to ``TRUE`` disables the package.
+
+* Setting the :variable:`CMAKE_REQUIRE_FIND_PACKAGE_<PackageName>` variable
+  to ``TRUE`` makes the package REQUIRED.
+
+Setting both variables to ``TRUE`` simultaneously is an error.
 
 Package File Interface Variables
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^