Right, I should probably have added the versions to the pkg-config check. Here
they are:

  libaccounts-glib 1.7
  libsignon-glib 1.8

As for the build error, here's what I get:
===========================================
In file included from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/util.h:25:0,
                 from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/IdentityProvider.h:22,
                 from src/backends/signon/signon.cpp:22:
src/backends/signon/signon.cpp: In member function 'virtual std::string
SyncEvo::SignonAuthProvider::getOAuth2Bearer(int) const':
src/backends/signon/signon.cpp:177:46: error: conversion from
'SyncEvo::PlainGStr' to 'bool' is ambiguous
                      buffer ? buffer.get() : "<<null>>",
                                              ^
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/Logging.h:454:43:
note: in definition of macro 'SE_LOG'
                                         ##_args);
                                           ^
src/backends/signon/signon.cpp:176:9: note: in expansion of macro
'SE_LOG_DEBUG'
         SE_LOG_DEBUG(NULL, "OAuth2 token result: %s, %s",
         ^
src/backends/signon/signon.cpp:166:19: note: candidates are:
         PlainGStr buffer(g_variant_print(sessionDataVar, true));
                   ^
In file included from src/backends/signon/signon.cpp:40:0:
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/GLibSupport.h:624:9:
note: SyncEvo::PlainGStr::operator const gchar*() const
         operator const gchar *() const { return &**this; }
         ^
In file included from /usr/include/boost/smart_ptr/shared_ptr.hpp:672:0,
                 from /usr/include/boost/shared_ptr.hpp:17,
                 from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/ThreadSupport.h:33,
                 from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/Logging.h:36,
                 from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/util.h:25,
                 from
/media/mardy/Data/src/git/freedesktop/syncevolution/src/syncevo/IdentityProvider.h:22,
                 from src/backends/signon/signon.cpp:22:
/usr/include/boost/smart_ptr/detail/operator_bool.hpp:52:5: note:
boost::shared_ptr<T>::operator boost::shared_ptr<T>::unspecified_bool_type()
const [with T = char; boost::shared_ptr<T>::unspecified_bool_type = char*
boost::shared_ptr<char>::*; boost::shared_ptr<T>::element_type = char]
     operator unspecified_bool_type() const BOOST_NOEXCEPT
     ^
===========================================