Description: Do not use pip to check sphinx rtd theme availability
Author: Francois Mazen <francois@mzf.fr>
Forwarded: https://github.com/ceres-solver/ceres-solver/issues/809

--- a/cmake/FindSphinx.cmake
+++ b/cmake/FindSphinx.cmake
@@ -50,7 +50,7 @@
   if(PYTHONINTERP_FOUND)
     # Check for sphinx theme dependency for documentation
     execute_process(
-      COMMAND ${PYTHON_EXECUTABLE} -m pip show sphinx-rtd-theme
+      COMMAND ${PYTHON_EXECUTABLE} -c "import sphinx_rtd_theme"
       RESULT_VARIABLE SPHINX_RTD_THEME
       OUTPUT_QUIET
       ERROR_QUIET