Fix dependency check for building documentation

Build with documentation fails if the required 'sphinx rtd theme' is
not available. Check if dependency is installed before building with
documentation.

Add Python3 as requirement for building documentation.

Change-Id: I5edc5d7374864990e625a5efb358f5a23b3c50fe

cmake/FindSphinx.cmake

@@ -43,24 +43,30 @@ find_program(SPHINX_EXECUTABLE
                /opt/local/bin
              DOC "Sphinx documentation generator")
 
-if (NOT SPHINX_EXECUTABLE)
-  set(_Python_VERSIONS 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.6 1.5)
+if (SPHINX_EXECUTABLE)
 
-  foreach (_version ${_Python_VERSIONS})
-    set(_sphinx_NAMES sphinx-build-${_version})
+  find_package(PythonInterp 3)
+
+  if(PYTHONINTERP_FOUND)
+    # Check for sphinx theme dependency for documentation
+    execute_process(
+      COMMAND ${PYTHON_EXECUTABLE} -m pip show sphinx-rtd-theme
+      RESULT_VARIABLE SPHINX_RTD_THEME
+      OUTPUT_QUIET
+      ERROR_QUIET
+    )
+  endif ()
+
+  if (${SPHINX_RTD_THEME} EQUAL 0)
+    set(SPHINX_RTD_THEME TRUE)
+  else ()
+    set(SPHINX_RTD_THEME FALSE)
+  endif ()
 
-    find_program(SPHINX_EXECUTABLE
-                 NAMES ${_sphinx_NAMES}
-                 PATHS
-                   /usr/bin
-                   /usr/local/bin
-                   /opt/local/bin
-                 DOC "Sphinx documentation generator")
-  endforeach ()
 endif ()
 
 include(FindPackageHandleStandardArgs)
 
-find_package_handle_standard_args(Sphinx DEFAULT_MSG SPHINX_EXECUTABLE)
+find_package_handle_standard_args(Sphinx DEFAULT_MSG SPHINX_EXECUTABLE SPHINX_RTD_THEME)
 
 mark_as_advanced(SPHINX_EXECUTABLE)