Fixing uninitialized PYTHON_INSTDIR in CMakeLists.txt (#652)

The variable PYTHON_INSTDIR is not initialized if the option
nanopb_BUILD_GENERATOR is OFF
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1b4f2d0..0975bc7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,17 +38,17 @@
     set(CMAKE_INSTALL_CMAKEDIR "${CMAKE_INSTALL_LIBDIR}/cmake/nanopb")
 endif()
 
+find_package(Python REQUIRED)
+execute_process(
+    COMMAND ${Python_EXECUTABLE} -c
+        "from distutils import sysconfig; print(sysconfig.get_python_lib(prefix=''))"
+    OUTPUT_VARIABLE PYTHON_INSTDIR
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
 if(nanopb_BUILD_GENERATOR)
     set(generator_protos nanopb)
 
-    find_package(Python REQUIRED)
-    execute_process(
-        COMMAND ${Python_EXECUTABLE} -c
-            "from distutils import sysconfig; print(sysconfig.get_python_lib(prefix=''))"
-        OUTPUT_VARIABLE PYTHON_INSTDIR
-        OUTPUT_STRIP_TRAILING_WHITESPACE
-    )
-
     foreach(generator_proto IN LISTS generator_protos)
         string(REGEX REPLACE "([^;]+)" "${PROJECT_SOURCE_DIR}/generator/proto/\\1.proto" generator_proto_file "${generator_proto}")
         string(REGEX REPLACE "([^;]+)" "\\1_pb2.py" generator_proto_py_file "${generator_proto}")