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, 14 insertions, 0 deletions

diff --git a/Help/variable/CMAKE_REQUIRE_FIND_PACKAGE_PackageName.rst b/Help/variable/CMAKE_REQUIRE_FIND_PACKAGE_PackageName.rst
new file mode 100644
index 0000000..893f1ae
--- /dev/null
+++ b/Help/variable/CMAKE_REQUIRE_FIND_PACKAGE_PackageName.rst
@@ -0,0 +1,14 @@
+CMAKE_REQUIRE_FIND_PACKAGE_<PackageName>
+----------------------------------------
+
+.. versionadded:: 3.22
+
+Variable for making :command:`find_package` call ``REQUIRED``.
+
+Every non-``REQUIRED`` :command:`find_package` call in a project can be
+turned into ``REQUIRED`` by setting the variable
+``CMAKE_REQUIRE_FIND_PACKAGE_<PackageName>`` to ``TRUE``.
+This can be used to assert assumptions about build environment and to
+ensure the build will fail early if they do not hold.
+
+See also the :variable:`CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` variable.