Fix compilation of triangulation code with CGAL

It is necessary to use -frounding-math to avoid run-time errors with
certain versions of CGAL.

configure.ac

@@ -179,6 +179,8 @@ dnl CGAL
 AC_CHECK_LIB(CGAL,main)
 [CGAL_LIBADD="-lCGAL"]
 AC_SUBST(CGAL_LIBADD)
+[CGAL_FLAGS="-frounding-math"]
+AC_SUBST(CGAL_FLAGS)
 
 dnl Checks for header files.
 

src/graph/generation/Makefile.am

@@ -2,7 +2,7 @@
 
 AM_CPPFLAGS = $(MOD_CPPFLAGS)
 
-AM_CXXFLAGS = $(CXXFLAGS)
+AM_CXXFLAGS = $(CXXFLAGS) $(CGAL_FLAGS)
 
 libgraph_tool_generationdir = $(pythondir)/graph_tool/generation