CMakeLists.txt: fix some inconsistent coding style

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

CMakeLists.txt

@@ -29,10 +29,10 @@ 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)
+    if(DOXYGEN_FOUND)
         # set input and output files
         set(DOXYGEN_IN ${CMAKE_CURRENT_SOURCE_DIR}/docs/Doxyfile.in)
         set(DOXYGEN_OUT ${CMAKE_CURRENT_BINARY_DIR}/Doxyfile)
@@ -47,7 +47,7 @@ if(BUILD_DOC)
             WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
             COMMENT "Generating API documentation with Doxygen"
             VERBATIM )
-    else (DOXYGEN_FOUND)
-      message("Doxygen need to be installed to generate the doxygen documentation")
-    endif (DOXYGEN_FOUND)
-endif()
+    else(DOXYGEN_FOUND)
+        message("Doxygen need to be installed to generate the doxygen documentation")
+    endif(DOXYGEN_FOUND)
+endif(BUILD_DOC)