Fix detection of sphinx-rtd-theme

Upstreaming fix from Debian.

https: //github.com/ceres-solver/ceres-solver/issues/809
Change-Id: I0e2f90a405a56ceffdda37f70d6e1ac853e176f1

cmake/FindSphinx.cmake

diff --git a/cmake/FindSphinx.cmake b/cmake/FindSphinx.cmake
index 2ba3c81..fa5925d 100644
--- 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