find_*: Add support for REQUIRED keyword

In the same spirit as the REQUIRED keyword on find_package, this will
stop cmake execution with an error on a failed find_program, find_file,
find_path or find_library.

Source/cmFindProgramCommand.cxx

@@ -3,6 +3,7 @@
 #include "cmFindProgramCommand.h"
 
 #include "cmMakefile.h"
+#include "cmMessageType.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
 #include "cmSystemTools.h"
@@ -136,6 +137,13 @@ bool cmFindProgramCommand::InitialPass(std::vector<std::string> const& argsIn)
   this->Makefile->AddCacheDefinition(
     this->VariableName, (this->VariableName + "-NOTFOUND").c_str(),
     this->VariableDocumentation.c_str(), cmStateEnums::FILEPATH);
+  if (this->Required) {
+    this->Makefile->IssueMessage(
+      MessageType::FATAL_ERROR,
+      "Could not find " + this->VariableName +
+        " using the following names: " + cmJoin(this->Names, ", "));
+    cmSystemTools::SetFatalErrorOccured();
+  }
   return true;
 }