build: don't fail pybind11 search if python is disabled (#4136)

Fixes #4135 

Thanks to @OgreTransporter for the suggestion.

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

src/cmake/externalpackages.cmake

@@ -158,8 +158,9 @@ endif()
 
 # From pythonutils.cmake
 find_python()
-
-checked_find_package (pybind11 REQUIRED VERSION_MIN 2.4.2)
+if (USE_PYTHON)
+    checked_find_package (pybind11 REQUIRED VERSION_MIN 2.4.2)
+endif ()
 
 
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