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, 21 insertions, 3 deletions

diff --git a/Source/cmFindPackageCommand.cxx b/Source/cmFindPackageCommand.cxx
index fba736eed6..a0de74c8dc 100644
--- a/Source/cmFindPackageCommand.cxx
+++ b/Source/cmFindPackageCommand.cxx
@@ -478,17 +478,35 @@ bool cmFindPackageCommand::InitialPass(std::vector<std::string> const& args)
       this->VersionMaxPatch, this->VersionMaxTweak);
   }
 
+  const std::string makePackageRequiredVar =
+    cmStrCat("CMAKE_REQUIRE_FIND_PACKAGE_", this->Name);
+  const bool makePackageRequiredSet =
+    this->Makefile->IsOn(makePackageRequiredVar);
+  if (makePackageRequiredSet) {
+    if (this->Required) {
+      this->Makefile->IssueMessage(
+        MessageType::WARNING,
+        cmStrCat("for module ", this->Name,
+                 " already called with REQUIRED, thus ",
+                 makePackageRequiredVar, " has no effect."));
+    } else {
+      this->Required = true;
+    }
+  }
+
   std::string disableFindPackageVar =
     cmStrCat("CMAKE_DISABLE_FIND_PACKAGE_", this->Name);
   if (this->Makefile->IsOn(disableFindPackageVar)) {
     if (this->Required) {
       this->SetError(
-        cmStrCat("for module ", this->Name, " called with REQUIRED, but ",
-                 disableFindPackageVar,
+        cmStrCat("for module ", this->Name,
+                 (makePackageRequiredSet
+                    ? " was made REQUIRED with " + makePackageRequiredVar
+                    : " called with REQUIRED, "),
+                 " but ", disableFindPackageVar,
                  " is enabled. A REQUIRED package cannot be disabled."));
       return false;
     }
-
     return true;
   }