Re-introduced the custom build defines
Re-introduced the custom build defines

The ability to use custom build defines were removed in commit:
1c5a0bb32583fd294022e68e66b541bf6ff71a67

This commit will reintroduce this feature, but using a prefixed variable
name.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml