Revert "configure: Disable -Wbool-operation."

This reverts commit c2d155e11ee5ec732d471982f2dee43703bcd5a7.

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’

1 files changed, 0 insertions, 1 deletions

diff --git a/configure b/configure
index f634a83bf6..0c75f7f5c9 100755
--- a/configure
+++ b/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