Add shenango runtime as a folly dependency

[saubhik]

To build folly, use the following commands.
Note that you might need to change those paths.
This command also builds tests.

cmake \
    -DCMAKE_INCLUDE_PATH=/proj/quic-server-PG0/users/saubhik/installed/googletest/include/ \
    -DCMAKE_LIBRARY_PATH=/proj/quic-server-PG0/users/saubhik/installed/googletest/lib/ \
    -DSHENANGO_RUNTIME_LIBRARY=/proj/quic-server-PG0/users/saubhik/caladan/bindings/cc/librt++.a \
    -DSHENANGO_RUNTIME_INCLUDE_DIR=/proj/quic-server-PG0/users/saubhik/caladan/bindings/cc \
    -DSHENANGO_BASE_LIBRARY=/proj/quic-server-PG0/users/saubhik/caladan/libbase.a \
    -DSHENANGO_BASE_INCLUDE_DIR=/proj/quic-server-PG0/users/saubhik/caladan/inc/ \
    -DCMAKE_INSTALL_PREFIX=/proj/quic-server-PG0/users/saubhik/installed/folly \
    -DBUILD_TESTS=ON .
make
make -j20

The build is successful with this commit.
Tests can be run as executables:

saubhik@node-0:/proj/quic-server-PG0/users/saubhik/folly$ ./ScopedEventBaseThreadTest 
[==========] Running 6 tests from 1 test case.
[----------] Global test environment set-up.
[----------] 6 tests from ScopedEventBaseThreadTest
[ RUN      ] ScopedEventBaseThreadTest.example
[       OK ] ScopedEventBaseThreadTest.example (0 ms)
[ RUN      ] ScopedEventBaseThreadTest.named_example
[       OK ] ScopedEventBaseThreadTest.named_example (1 ms)
[ RUN      ] ScopedEventBaseThreadTest.default_manager
[       OK ] ScopedEventBaseThreadTest.default_manager (0 ms)
[ RUN      ] ScopedEventBaseThreadTest.custom_manager
[       OK ] ScopedEventBaseThreadTest.custom_manager (0 ms)
[ RUN      ] ScopedEventBaseThreadTest.eb_dtor_in_io_thread
[       OK ] ScopedEventBaseThreadTest.eb_dtor_in_io_thread (1 ms)
[ RUN      ] ScopedEventBaseThreadTest.keepalive
[       OK ] ScopedEventBaseThreadTest.keepalive (0 ms)
[----------] 6 tests from ScopedEventBaseThreadTest (2 ms total)

[----------] Global test environment tear-down
[==========] 6 tests from 1 test case ran. (2 ms total)
[  PASSED  ] 6 tests.