CPack/NSIS: Add option for setting custom makensis executable

It allows users to set a custom one without changing the PATH environment variable.

Fixes: #21733

Help/cpack_gen/nsis.rst

@@ -193,3 +193,9 @@ on Windows Nullsoft Scriptable Install System.
  .. versionadded:: 3.20
 
  If set, trim down the size of the control to the size of the branding text string.
+
+.. variable:: CPACK_NSIS_EXECUTABLE
+
+ .. versionadded:: 3.21
+
+ If set, specify the name of the NSIS executable. Default is ``makensis``.

Help/release/dev/cpack-nsis-executable-name.rst

@@ -0,0 +1,6 @@
+cpack-nsis-executable-name
+--------------------------
+
+* The :cpack_gen:`CPack NSIS Generator` gained a new variable
+  :variable:`CPACK_NSIS_EXECUTABLE` to specify the makensis
+  executable to use instead of the default one.

Source/CPack/cmCPackNSISGenerator.cxx

@@ -430,7 +430,9 @@ int cmCPackNSISGenerator::InitializeInternal()
   }
 #endif
 
-  nsisPath = cmSystemTools::FindProgram("makensis", path, false);
+  this->SetOptionIfNotSet("CPACK_NSIS_EXECUTABLE", "makensis");
+  nsisPath = cmSystemTools::FindProgram(
+    this->GetOption("CPACK_NSIS_EXECUTABLE"), path, false);
 
   if (nsisPath.empty()) {
     cmCPackLogger(