Sphinx is not required for building any more

docs/CMakeLists.txt

@@ -1,31 +1,40 @@
-find_package(Sphinx REQUIRED)
- 
-if(NOT DEFINED SPHINX_THEME)
-    set(SPHINX_THEME default)
+# cmake should not fail if sphinx is missing
+find_package(Sphinx)
+
+if(SPHINX_EXECUTABLE)
+     
+    if(NOT DEFINED SPHINX_THEME)
+        set(SPHINX_THEME default)
+    endif()
+     
+    if(NOT DEFINED SPHINX_THEME_DIR)
+        set(SPHINX_THEME_DIR)
+    endif()
+     
+    # configured documentation tools and intermediate build results
+    set(BINARY_BUILD_DIR "${CMAKE_CURRENT_BINARY_DIR}/_build")
+     
+    # Sphinx cache with pickled ReST documents
+    set(SPHINX_CACHE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_doctrees")
+     
+    # HTML output directory
+    set(SPHINX_HTML_DIR "${CMAKE_CURRENT_BINARY_DIR}/html")
+     
+    configure_file(
+        "${CMAKE_CURRENT_SOURCE_DIR}/conf.py"
+        "${BINARY_BUILD_DIR}/conf.py"
+        @ONLY)
+     
+    add_custom_target(docs
+        ${SPHINX_EXECUTABLE}
+        -b html
+        -c "${BINARY_BUILD_DIR}"
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+        "${SPHINX_HTML_DIR}"
+        COMMENT "Building HTML documentation with Sphinx")
+else()
+    add_custom_target(docs
+        echo
+        "Please install python-sphinx to build the docs or read the docs online: https://projecttox.readthedocs.org/en/latest"
+        COMMENT "No sphinx executebale found")
 endif()
- 
-if(NOT DEFINED SPHINX_THEME_DIR)
-    set(SPHINX_THEME_DIR)
-endif()
- 
-# configured documentation tools and intermediate build results
-set(BINARY_BUILD_DIR "${CMAKE_CURRENT_BINARY_DIR}/_build")
- 
-# Sphinx cache with pickled ReST documents
-set(SPHINX_CACHE_DIR "${CMAKE_CURRENT_BINARY_DIR}/_doctrees")
- 
-# HTML output directory
-set(SPHINX_HTML_DIR "${CMAKE_CURRENT_BINARY_DIR}/html")
- 
-configure_file(
-    "${CMAKE_CURRENT_SOURCE_DIR}/conf.py"
-    "${BINARY_BUILD_DIR}/conf.py"
-    @ONLY)
- 
-add_custom_target(docs
-    ${SPHINX_EXECUTABLE}
-    -b html
-    -c "${BINARY_BUILD_DIR}"
-    "${CMAKE_CURRENT_SOURCE_DIR}"
-    "${SPHINX_HTML_DIR}"
-    COMMENT "Building HTML documentation with Sphinx")