This must be about gnash's configure.ac file.
The slackbuild script uses autoreconf don't include -lboost_system.
I think so because configure and compiling works on the git tree.
aclocal.m4:16: warning: this file was generated for autoconf 2.64.
You have another version of autoconf. It may work, but is not guaranteed to.
If you have problems, you may need to regenerate the build system entirely.
To do so, use the procedure documented by the package, typically `autoreconf'.
I must rewrite the buildscrip for a git version