boost program options: win64: bcp namespace renamed version: *.lib file of program_options for *.dll is not created

Between boost 1.63.0 and 1.65.1 line 12 of file 'libs/program_options/build/Jamfile.v2' has been changed:
https://github.com/boostorg/program_options/commit/1f9413f532e58b82d12e9185c2eb8bc45612914c

This change is the reason why the *.lib file for the program_options *.dll file is not created on win64/win32.

Please consider reverting the change mentioned above partially and put back the definition of the flag BOOST_PROGRAM_OPTIONS_DYN_LINK=1:
```
diff -ruN boost_1_65_1.orig/libs/program_options/build/Jamfile.v2 boost_1_65_1/libs/program_options/build/Jamfile.v2
--- boost_1_65_1.orig/libs/program_options/build/Jamfile.v2 2018-11-20 15:18:48.469951300 +0100
+++ boost_1_65_1/libs/program_options/build/Jamfile.v2 2018-11-22 10:27:15.108147800 +0100
@@ -11,6 +11,10 @@

boost-lib program_options
: $(SOURCES).cpp
- : # See https://svn.boost.org/trac/boost/ticket/5049
+ : shared:BOOST_PROGRAM_OPTIONS_DYN_LINK=1 # tell source we're building dll's
+ # See https://svn.boost.org/trac/boost/ticket/5049
hpux,gcc:_INCLUDE_STDC__SOURCE_199901
- ;
\ No newline at end of file
+ :
+ : shared:BOOST_PROGRAM_OPTIONS_DYN_LINK=1
+ ;
+
```
Similar to issue: https://github.com/boostorg/program_options/issues/60

Inizia da libs/program_options/build/Jamfile.v2 e confronta il commit indicato e l’issue 60. Riproduci una build Windows shared per program_options con la toolchain Boost supportata, quindi verifica che la libreria di importazione corrispondente venga generata insieme alla DLL; l’issue è completata quando questo funziona per win32 e win64.

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