[warningectomy] unused const variable

node.c:58:21: warning: unused variable 'NODE_BORDERWIDTH'
      [-Wunused-const-variable]
static const double NODE_BORDERWIDTH = 0.1;
                    ^
node.c:60:21: warning: unused variable 'NODE_FONTHEIGHT'
      [-Wunused-const-variable]
static const double NODE_FONTHEIGHT = 0.8;
                    ^
2 warnings generated.
Bug 737254 - Use AC_CONFIG_HEADERS instead of AC_CONFIG_HEADER

Could not find any documentation for the later, we can not use
AM_CONFIG_HEADER long time deprecated, finally gone.