qmake & windows: how to convert backslashes into forward slashes -

i'm trying utilize bison, i've specified next line: bison.commands = bison -d -o ${qmake_file_path}/${qmake_file_base}.parser.cc ${qmake_file_in}

the problem ${qmake_file_in} , ${qmake_file_path} contain backslashes bison doesn't understand. how convert them forwards slashes?

windows qmake