[doc] include Doxygen comments for HAVE_BOOST_PROCESS

Author: Sjors

doc/Doxyfile.in

@@ -2073,7 +2073,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             =
+PREDEFINED             = HAVE_BOOST_PROCESS
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The