build error with boost 1.68

When building boost 1.68, I have an error when building program_options where it cannot copy the program_options-mt.lib file.

It seems the .lib file is not created since nothing is exported when not using `BOOST_ALL_DYN_LIB`.

Taking example on boost.timer, I modified the Jamfile.v2 file to look like this
```
boost-lib program_options
: $(SOURCES).cpp
: # See https://svn.boost.org/trac/boost/ticket/5049
hpux,gcc:_INCLUDE_STDC__SOURCE_199901
shared:BOOST_PROGRAM_OPTIONS_DYN_LINK=1
static:BOOST_PROGRAM_OPTIONS_STATIC_LINK=1
;
```

and it seems to fix my issue.

Start with the program_options Jamfile.v2 and compare its boost-lib definition with the boost.timer example mentioned in the issue. Reproduce the Boost 1.68 build and verify that program_options-mt.lib is created and can be copied successfully.

Scritto dal modello di indicizzazione a partire dal testo della issue.