[build] fix issue when executing build-commissioner-libs.sh multipl…

…e times (#250)

When developers run the `build-commissioner-libs.sh` script multiple
times, they might see the following error during compiling the
libevent:

error: call to undeclared function 'pipe2'; ISO C99 and later do not
support implicit function declarations ...

In fact, we did not use pip2 in libevent. We can turn it off by using
the macro EVENT_HAVE-PIPE2=0

third_party/libevent/CMakeLists.txt

@@ -36,6 +36,7 @@ set(EVENT__DISABLE_SAMPLES ON CACHE BOOL "libevent disable samples" FORCE)
 set(EVENT__DISABLE_REGRESS ON CACHE BOOL "libevent disable regression tests" FORCE)
 set(EVENT__DISABLE_TESTS ON CACHE BOOL "libevent disable tests" FORCE)
 set(EVENT__DISABLE_BENCHMARK ON CACHE BOOL "libevent disable benchmark" FORCE)
+set(EVENT__HAVE_PIPE2 OFF CACHE BOOL "disable use of pipe2 in libevent" FORCE)
 add_subdirectory(repo)
 
 target_include_directories(event_core