- Change pkg-config file to use ${prefix}/include

rather thant ${prefix}/include/sgpemv2: users will
be able to use directives like "#include <sgpemv2/scheduler.hh>"
in their headers, which reduces name clashes.
- Make _mutex a protected member of Singleton, so that
inheriters can recycle it :-)
- Add exclusive access control for step_forward()
- Add two states (future and exhausted) to Request


git-svn-id: svn://svn.gna.org/svn/sgpemv2/trunk@707 3ecf2c5c-341e-0410-92b4-d18e462d057c

config/sgpemv2.pc.in

@@ -14,4 +14,4 @@ URL: http://www.math.unipd.it/
 Requires: glibmm-2.4 >= 2.8  gthread-2.0 >= 2.8
 Libs: -L${libdir} -lbackend
 Libs.private: -lglibmm-2.4 -lgthread-2.0
-Cflags: -I${includedir}/sgpemv2
+Cflags: -I${includedir}