[LyX/master] Instantiate trivial_string::c_str() explicitly

[Georg Baum]

commit b147cd0f3a2b69cd4a983d1b35f29fa7502855f3
Author: Georg Baum <b...@lyx.org>
Date:   Fri May 15 21:01:59 2015 +0200

    Instantiate trivial_string::c_str() explicitly
    
    This fixes hopefully the compilation of check_trivstring on cygwin and
    solaris. Previously I did not use the explicit instantiation, because 
c_str()
    is used by other explicitly instantiated methiods, but for some reason this
    does not seem to suffice.

diff --git a/src/support/trivstring.cpp b/src/support/trivstring.cpp
index c780aa7..127a8d3 100644
--- a/src/support/trivstring.cpp
+++ b/src/support/trivstring.cpp
@@ -187,6 +187,8 @@ trivial_string<Char>::operator _stdstring() const
 }
 
 
+template char const * trivial_string<char>::c_str() const;
+template char_type const * trivial_string<char_type>::c_str() const;
 template<typename Char> Char const * trivial_string<Char>::c_str() const
 {
        if (use_sso())