[build] make boost-process opt-in

Author: Sjors

configure.ac

@@ -1190,7 +1190,9 @@ fi
 AX_BOOST_SYSTEM
 AX_BOOST_FILESYSTEM
 AX_BOOST_THREAD
-AX_BOOST_PROCESS
+
+dnl Opt-in to boost-process
+AS_IF([ test x$with_boost_process != x ], [ AX_BOOST_PROCESS ], [ ax_cv_boost_process=no ] )
 
 dnl Boost 1.56 through 1.62 allow using std::atomic instead of its own atomic
 dnl counter implementations. In 1.63 and later the std::atomic approach is default.