Revert to the canonical way of customizing CXXFLAGS

A default list of compiler flags is set via AM_CXXFLAGS Automake variable.
However this gets overridden by per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS in case the foo target). Because of this you should append
$(AM_CXXFLAGS) to the end of the per-target CXXFLAGS variables (e.g.
foo_CXXFLAGS = ... $(AM_CXXFLAGS)).

After this default list of flags the contents of the CXXFLAGS user variable is
passed to the compiler. This variable has a default value determined by
configure (in case of g++ it is '-g -O2'). You can override this by specifying
CXXFLAGS when invoking make (e.g. make CXXFLAGS='-O3').

 @nodes

 label	supply1	supply2	supply3

 1	0	20	27	

 2	0	-4	0		

 3	0	0	0	

 4	0	0	0	

 5	0	9	0	

 6	0	-6	0	

 7	0	0	0	

 8	0	0	0	

 9	0	3	0	

 10	0	-2	0	

 11	0	0	0		

 12	0	-20	-27	

 @arcs

 		cost	capacity	lower1	lower2

 1	2	70	11		0	8

 1	3	150	3		0	1

 1	4	80	15		0	2

 2	8	80	12		0	0

 3	5	140	5		0	3

 4	6	60	10		0	1

 4	7	80	2		0	0

 4	8	110	3		0	0

 5	7	60	14		0	0

 5	11	120	12		0	0

 6	3	0	3		0	0

 6	9	140	4		0	0

 6	10	90	8		0	0

 7	1	30	5		0	0

 8	12	60	16		0	4

 9	12	50	6		0	0

 10	12	70	13		0	5

 10	2	100	7		0	0

 10	7	60	10		0	0

 11	10	20	14		0	6

 12	11	30	10		0	0

 @attributes

 source	1

 target	12

 @end