CMakeLists.txt: fix some inconsistent coding style

Signed-off-by: Ming Liu <liu.ming50@gmail.com>

CMakeLists.txt

@@ -29,8 +29,8 @@ add_subdirectory (src)
 # first we can indicate the documentation build as an option and set it to ON by default
 option(BUILD_DOC "Build documentation" ON)
 
-# check if Doxygen is installed
 if(BUILD_DOC)
+    # check if Doxygen is installed
     find_package(Doxygen)
     if(DOXYGEN_FOUND)
         # set input and output files
@@ -50,4 +50,4 @@ if(BUILD_DOC)
     else(DOXYGEN_FOUND)
         message("Doxygen need to be installed to generate the doxygen documentation")
     endif(DOXYGEN_FOUND)
-endif()
+endif(BUILD_DOC)