Build failure with Boost 1.58 on Linux

when I try to use Boost 1.58, I get the following error:

```
$ make check

Making check in .
make[1]: Entering directory '/home/fifilyu/workspace/folly/folly'
make[1]: Leaving directory '/home/fifilyu/workspace/folly/folly'
Making check in test
make[1]: Entering directory '/home/fifilyu/workspace/folly/folly/test'
Making check in .
make[2]: Entering directory '/home/fifilyu/workspace/folly/folly/test'
make  libgtestmain.la libgtest.la benchmark_test concurrent_skiplist_benchmark sorted_vector_types_test foreach_test hash_test timeout_queue_test conv_test range_test bits_test bit_iterator_test spin_lock_test small_locks_test packed_sync_ptr_test small_vector_test discriminated_ptr_test cpuid_test fbstring_test_using_jemalloc thread_cached_int_test thread_local_test fbvector_test dynamic_test json_test scope_guard_test endian_test rw_spinlock_test synchronized_test concurrent_skiplist_test histogram_test group_varint_test map_util_test string_test producer_consumer_queue_test atomic_hash_array_test atomic_hash_map_test format_test fingerprint_test portability_test spooky_hash_v1_test spooky_hash_v2_test
make[3]: Entering directory '/home/fifilyu/workspace/folly/folly/test'
make[3]: 'libgtestmain.la' is up to date.
make[3]: 'libgtest.la' is up to date.
/bin/sh ../libtool  --tag=CXX   --mode=link g++  -std=gnu++0x -g -O2 -lboost_thread -lboost_filesystem -lboost_system -lboost_regex -lpthread  -o benchmark_test BenchmarkTest.o libgtestmain.la ../libfollybenchmark.la ../libfolly.la -llzma -lz -lsnappy -llz4 -liberty -ljemalloc -levent -ldouble-conversion -lssl -lgflags -lglog 
libtool: link: g++ -std=gnu++0x -g -O2 -o .libs/benchmark_test BenchmarkTest.o  ./.libs/libgtestmain.a ../.libs/libfollybenchmark.so /home/fifilyu/workspace/folly/folly/.libs/libfolly.so ../.libs/libfolly.so -lboost_thread -lboost_filesystem -lboost_system -lboost_regex -lpthread -llzma -lz -lsnappy -llz4 -liberty -ljemalloc -levent -ldouble-conversion -lssl -lgflags -lglog -Wl,-rpath -Wl,/usr/local/folly/lib
/home/fifilyu/workspace/folly/folly/.libs/libfolly.so: undefined reference to `jump_fcontext'
/home/fifilyu/workspace/folly/folly/.libs/libfolly.so: undefined reference to `make_fcontext'
collect2: error: ld returned 1 exit status
Makefile:1024: recipe for target 'benchmark_test' failed
make[3]: *** [benchmark_test] Error 1
make[3]: Leaving directory '/home/fifilyu/workspace/folly/folly/test'
Makefile:1868: recipe for target 'check-am' failed
make[2]: *** [check-am] Error 2
make[2]: Leaving directory '/home/fifilyu/workspace/folly/folly/test'
Makefile:1305: recipe for target 'check-recursive' failed
make[1]: *** [check-recursive] Error 1
make[1]: Leaving directory '/home/fifilyu/workspace/folly/folly/test'
Makefile:1448: recipe for target 'check-recursive' failed
make: *** [check-recursive] Error 1

```

the message's full context is visible @ https://gist.github.com/fifilyu/9ef059a9a17811cc54d6

fix for this error:

```
diff --git a/folly/m4/ax_boost_context.m4 b/folly/m4

[]