find: use sysconf(_SC_ARG_MAX) to determine the command-line size limit

The find utility uses a hardcoded value of 32 * 1024 as the limit of
the command-line length when calling 'find -exec ... {} +'. This results
in over 4 times more execve() calls than in coreutils' find.

This patch uses the limit defined in system headers.

Based on the patch by  Bartosz Golaszewski <bartekgola@gmail.com>.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/Kbuild.src

@@ -92,6 +92,7 @@ lib-y += skip_whitespace.o
 lib-y += speed_table.o
 lib-y += str_tolower.o
 lib-y += strrstr.o
+lib-y += sysconf.o
 lib-y += time.o
 lib-y += trim.o
 lib-y += u_signal_names.o