find sphinx-build before calling build.py

this allows us to use sphinx-build-3, as user might already in a python3
world, and distros, like fedora 28, tend to add a "-3" suffix to sphinx-build.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

doc/CMakeLists.txt

@@ -4,8 +4,11 @@ if (NOT DOXYGEN)
   return ()
 endif ()
 
+find_program(SPHINX_EXECUTABLE
+  NAMES sphinx-build sphinx-build-3)
 add_custom_target(doc
-  COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
+  COMMAND ${CMAKE_COMMAND} -E env SPHINX_EXECUTABLE=${SPHINX_EXECUTABLE}
+    ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION})
   SOURCES api.rst syntax.rst build.py conf.py _templates/layout.html)
 
 install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/