Revert "configure: Disable -Wbool-operation."

This reverts commit c2d155e1.

GCC 6 incorrectly passes the configure test and then logs many warnings
of the form:

src/libavformat/dump.c: At top level:
cc1: warning: unrecognized command line option ‘-Wno-bool-operation’

configure

@@ -6297,7 +6297,6 @@ check_cflags -Wmissing-prototypes
 check_cflags -Wno-pointer-to-int-cast
 check_cflags -Wstrict-prototypes
 check_cflags -Wempty-body
-check_cflags -Wno-bool-operation
 
 if enabled extra_warnings; then
     check_cflags -Wcast-qual