[gnome-sudoku] Link with pthreads

[Ernestas Kulik <ernestask src gnome org>]

commit ec20e8433b84e0aee9f51518a18b79780082cf5e
Author: Ernestas Kulik <ernestask gnome org>
Date:   Wed Nov 29 14:12:32 2017 +0200

    Link with pthreads
    
    Calling std::call_once() from the C++11 thread support library without having
    linked to a pthread library will result in an unhandled exception when
    generating puzzles.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=790981

 configure.ac    |    2 ++
 lib/Makefile.am |    5 +++++
 2 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 9f6042c..4b119f8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -54,6 +54,8 @@ PKG_CHECK_MODULES(LIBSUDOKU, [
 
 AC_SUBST([GLIB_REQUIRED])
 
+AX_PTHREAD
+
 AC_PATH_PROG([DESKTOP_FILE_VALIDATE], [desktop-file-validate], [/bin/true])
 
 AC_CHECK_LIB([m],[floor])
diff --git a/lib/Makefile.am b/lib/Makefile.am
index b994bb4..5cd65fa 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -17,7 +17,12 @@ libsudoku_la_CPPFLAGS = \
        -DGETTEXT_PACKAGE=\"$(GETTEXT_PACKAGE)\" \
        $(LIBSUDOKU_CFLAGS)
 
+# C++11 thread support library needs pthreads to work properly.
+libsudoku_la_CXXFLAGS = \
+       $(PTHREAD_CFLAGS)
+
 libsudoku_la_LIBADD = \
+       $(PTHREAD_LIBS) \
        $(LIBSUDOKU_LIBS)
 
 libsudoku_la_VALAFLAGS = \