title: Autotools : how to set global compilation flag
body: <p>I have a project with several sources directories : </p>
  
  <pre><code>src/A
     /B
     /C
  </code></pre>
  
  <p>In each, the Makefile.am contains </p>
  
  <pre><code>AM_CXXFLAGS = -fPIC -Wall -Wextra
  </code></pre>
  
  <p>How can avoid repeating this in each source folder ? </p>
  
  <p>I tried to modifiy src/Makefile.am and the configure.in, but without success. I thought I could use AC_PROG_CXX to set the compilation flags globally but can't find much documentation on how to use those macro (do you have any pointer to such a documentation ?).</p>
  
  <p>Thanks in advance</p>