build: Adjust pystring finding (#4816)

We have been having intermittent CI failures when the auto-build of
OpenColorIO triggers the auto-build of Pystring, and then sometimes
immediately seems to think the version it installed is not correct.

I'm jiggling some things here to see if I can get it to stop
occasionally failing. I've run the testsuite on this patch several times
without seeing any failures that in recent days had been happening
typically in 1-2 jobs per CI run. So maybe this fixes it? I'd be lying
if I claimed to have a coherent and full explanation for exactly why
these changes help.

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

Author: lgritz

src/cmake/build_pystring.cmake

@@ -30,11 +30,13 @@ build_dependency_with_cmake(pystring
 )
 
 set (pystring_VERSION ${pystring_BUILD_VERSION})
+unset (PYSTRING_LIBRARY)
+unset (PYSTRING_INCLUDE_DIR)
 
-set (pystring_REFIND TRUE)
+set (pystring_REFIND FALSE)
 set (pystring_REFIND_VERSION ${pystring_BUILD_VERSION})
 
 
 if (pystring_BUILD_SHARED_LIBS)
     install_local_dependency_libs (pystring pystring)
-endif ()
+endif ()