Especially useful when cross compiling and when enabling gr-filter
feature, which has the following in upstream's CMakeLists.txt file:
```
gr_python_check_module_raw("pyqtgraph" "import pyqtgraph" PYQTGRAPH_FOUND)
gr_python_check_module_raw("scipy" "import scipy" SCIPY_FOUND)
if(NOT PYQTGRAPH_FOUND OR NOT SCIPY_FOUND)
message(WARNING "PyQtGraph and Scipy are required to run the filter design tool, "
"but are not detected! Please make sure they are installed on "
"the target system.")
endif(NOT PYQTGRAPH_FOUND OR NOT SCIPY_FOUND)
if(ENABLE_PYTHON)
add_subdirectory(python/filter)
add_subdirectory(python/filter/design)
add_subdirectory(python/filter/gui)
add_subdirectory(apps)
if(ENABLE_EXAMPLES)
add_subdirectory(examples)
endif(ENABLE_EXAMPLES)
endif(ENABLE_PYTHON)
if(ENABLE_GRC)
add_subdirectory(grc)
endif(ENABLE_GRC)
```
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