Ninja,Makefile: Drop extra space after RULE_LAUNCH_CUSTOM value

The Ninja and Makefile generators were adding a space to the value both
before and after expanding rule variables.  Only the latter is needed.

While at it, revise some outdated comments since the rule variable
expansion is no longer responsible for inserting the launcher.

Suggested-by: Mate Pek <[email protected]>
Fixes: #18340

Source/cmLocalNinjaGenerator.cxx

@@ -564,8 +564,7 @@ std::string cmLocalNinjaGenerator::MakeCustomLauncher(
     return std::string();
   }
 
-  // Expand rules in the empty string.  It may insert the launcher and
-  // perform replacements.
+  // Expand rule variables referenced in the given launcher command.
   cmRulePlaceholderExpander::RuleVariables vars;
 
   std::string output;
@@ -580,12 +579,10 @@ std::string cmLocalNinjaGenerator::MakeCustomLauncher(
   }
   vars.Output = output.c_str();
 
-  std::string launcher = property_value;
-  launcher += " ";
-
   std::unique_ptr<cmRulePlaceholderExpander> rulePlaceholderExpander(
     this->CreateRulePlaceholderExpander());
 
+  std::string launcher = property_value;
   rulePlaceholderExpander->ExpandRuleVariables(this, launcher, vars);
   if (!launcher.empty()) {
     launcher += " ";

Source/cmLocalUnixMakefileGenerator3.cxx

@@ -967,8 +967,7 @@ void cmLocalUnixMakefileGenerator3::AppendCustomCommand(
       // Short-circuit if there is no launcher.
       const char* val = this->GetRuleLauncher(target, "RULE_LAUNCH_CUSTOM");
       if (val && *val) {
-        // Expand rules in the empty string.  It may insert the launcher and
-        // perform replacements.
+        // Expand rule variables referenced in the given launcher command.
         cmRulePlaceholderExpander::RuleVariables vars;
         vars.CMTargetName = target->GetName().c_str();
         vars.CMTargetType = cmState::GetTargetTypeName(target->GetType());
@@ -986,7 +985,6 @@ void cmLocalUnixMakefileGenerator3::AppendCustomCommand(
         vars.Output = output.c_str();
 
         launcher = val;
-        launcher += " ";
         rulePlaceholderExpander->ExpandRuleVariables(this, launcher, vars);
         if (!launcher.empty()) {
           launcher += " ";