title: add_custom_target SOURCES option adds dependency to MAIN_DEPENDENCY custom commands
body: <p>Given the following CMakeLists.txt and some arbitrary hello.c:</p>
  
  <pre><code>cmake_minimum_required(VERSION 2.6)
  
  get_filename_component(debug_rpath ${CMAKE_CURRENT_BINARY_DIR}/hello-debug.exe REALPATH)
  message(STATUS "debug exe: ${debug_rpath}")
  get_filename_component(release_rpath ${CMAKE_CURRENT_BINARY_DIR}/hello-release.exe REALPATH)
  message(STATUS "release exe: ${release_rpath}")
  
  add_custom_command(
    OUTPUT hello-release.exe
    COMMAND clang ${CMAKE_SOURCE_DIR}/hello.c -O3 -o ${release_rpath}
    MAIN_DEPENDENCY hello.c
  #  DEPENDS hello.c
    IMPLICIT_DEPENDS hello.c
    COMMENT "Compiling release..."
    VERBATIM)
  
  add_custom_command(
    OUTPUT ${debug_rpath}
    COMMAND clang ${CMAKE_SOURCE_DIR}/hello.c -g -DNDEBUG -o ${debug_rpath}
    MAIN_DEPENDENCY hello.c
  #  DEPENDS hello.c
    IMPLICIT_DEPENDS hello.c
    COMMENT "Compiling debug..."
    VERBATIM)
  
  
  add_custom_target(hello-release ALL
    DEPENDS ${release_rpath}
    SOURCES hello.c
    VERBATIM)
  
  
  add_custom_target(hello-debug ALL
    DEPENDS ${debug_rpath}
    SOURCES hello.c
    VERBATIM)
  </code></pre>
  
  <p>(debug/release and clang as compiler are examples, I know cmake has its own mechanism for that. The important detail is that I have two custom commands using the the same main input file)</p>
  
  <p>Executing the <code>hello-debug</code> target outputs:</p>
  
  <pre><code>&gt; mingw32-make hello-debug
  [ 50%] Compiling release...
  [100%] Compiling debug...
  [100%] Built target hello-debug
  </code></pre>
  
  <p>Why does it also build release? </p>
  
  <p>Uncommenting the <code>SOURCES</code> or <code>MAIN_DEPENDENCY</code> options builds just the chosen target. I understand the documentation like the two options only influence Visual Studio project generation. I use "MinGW Makefiles" and "Visual Studio" generators at the same time, therefore keeping the SOURCES option would actually be nice. Bug or am I overseeing something?</p>