title: cmake seems to not pick up my -DCMAKE_CXX_FLAGS
body: <p>I have a totally simple setup. Two files in two separate directories.</p>
  
  <pre><code>mkdir a
  touch a/a.h
  mkdir b
  echo '#include &lt;a/a.h&gt;' &gt; b/b.c
  </code></pre>
  
  <p>Compiling works, when I specify a header path</p>
  
  <pre><code>cd b
  gcc -c -I.. b.c
  cd ..
  </code></pre>
  
  <p>OK now let's add <strong>cmake</strong> to the picture. For my purposes I need to specify the header search path via the command-line. Consider the CMakeLists.txt read only.</p>
  
  <pre><code>cat&lt;&lt;EOF &gt; b/CMakeLists.txt
  cmake_minimum_required (VERSION 3.0)
  
  project (b)
  
  add_library(b
  b.c
  )
  EOF
  
  
  mkdir b/build
  cd b/build
  cmake -DCMAKE_CXX_FLAGS=-I.. ..
  make VERBOSE=1
  </code></pre>
  
  <p>But make fails and I don't see the <strong>-I..</strong> specification in the <code>cc</code> command line. </p>
  
  <pre><code>[ 50%] Building C object CMakeFiles/b.dir/b.c.o
  /Applications/Xcode.app/Contents/Developer/Toolchains    /XcodeDefault.xctoolchain/usr/bin/cc    -o CMakeFiles/b.dir/b.c.o   -c /tmp/b/b.c
  /tmp/b/b.c:1:10: fatal error: 'a/a.h' file not found
  </code></pre>
  
  <p>I tried giving an absolute path too, but it just doesn't work for me. </p>