check for member scolbga in class plstream

[Peter Johansson]

I have an old pkg-config (0.22) and an old version of plplot (5.8).

Here

./configure 

passes and

make

fails with

../../lib/Graph.cc: In constructor ‘theplu::svndigest::Graph::Graph(const std::string&, const std::string&)’:
../../lib/Graph.cc:48: error: ‘class plstream’ has no member named ‘scolbga’
../../lib/Graph.cc: In member function ‘void theplu::svndigest::Graph::current_color(const theplu::svndigest::Graph::legend_data&)’:

I think this could get improved. The reason configure goes through here is that the version test is turned off because pkg-config (0.23) is not available. Perhaps we can modify the compiler test so that the compiler test fails when

 ‘class plstream’ has no member named ‘scolbga’

[Peter Johansson]

(In [1029]) fixes #446