Retain terminal formatting when building docs

This prevents Sphinx output to be stripped of colors, emphasis etc.

Change-Id: I127e02cbdda69a5d49a73678a7a7e2b4512b189e

docs/source/CMakeLists.txt

diff --git a/docs/source/CMakeLists.txt b/docs/source/CMakeLists.txt
index cb4b7df..2b6d749 100644
--- a/docs/source/CMakeLists.txt
+++ b/docs/source/CMakeLists.txt
@@ -19,4 +19,5 @@
                   "${Ceres_SOURCE_DIR}"
                   "${Ceres_BINARY_DIR}/docs"
                   "${SPHINX_EXECUTABLE}"
+                  USES_TERMINAL
                   COMMENT "Building HTML documentation with Sphinx")