Build system: improvements to Meson cross-file for Windows

– enforce POSIX version of the MinGW cross-compiler
– add strip binary

[skip ci]

windows-cross.ini

@@ -2,7 +2,8 @@
 # NB: The boost_root property must be set, possibly through a second cross file
 
 [binaries]
-cpp = 'x86_64-w64-mingw32-g++'
+cpp = 'x86_64-w64-mingw32-g++-posix'
+strip = 'x86_64-w64-mingw32-strip'
 
 [host_machine]
 system = 'windows'