synergy: Fix build with gcc 6

synergy: Fix build with gcc 6

Fixes:
http://autobuild.buildroot.net/results/05b/05bbc22dd6cd5564462226e612ab1e95778fe935/
http://autobuild.buildroot.net/results/edf/edfe58749d1b7a1bb2e0184a6824a74b9d38ddb9/
http://autobuild.buildroot.net/results/139/1395eca13ca537bde928ddd68a5bc6e130e82ba3/
http://autobuild.buildroot.net/results/94a/94ad6e8bbbeb926ea834d9d6e3ba87d0398acb86/

Gcc 6.x defaults to C++14, and the iostream operator bool behaviour changed
in C++11.  In previous versions, a somewhat odd operator void* was used to
return the status of the stream as a pointer.  Since C++11 a more sensible
operator bool is used to return the stream staus.

For details, see:

http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool

The code in CConfigReadContext assumes the pre-C++11 behaviour and provides
its own operator void overload to return the status of the embedded
iostream.  With C++11, iostream no longer provides this overload, breaking
the build:

CConfig.cpp: In member function 'CConfigReadContext::operator void*() const':
CConfig.cpp:1851:9: error: cannot convert 'std::istream {aka std::basic_istream<char>}' to 'void*' in return
  return m_stream;

To fix it, backport part of upstream commit 3d963bfbe7897d0a33ad (possible
fix for mavericks) which changes the code to simply provide a getStream()
method which returns a reference to the embedded stream and the calling code
is changed to use operator bool on the returned stream, making the code
compatible with both old and new compilers.

This upstream commit is part of the 1.6.0 release, so can be dropped when
the version is bumped.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>