Bug#846435: libmatheval: add libfl-dev to Build-Depends

[Helmut Grohne]

Source: libmatheval
Severity: important
User: helmutg at debian.org
Usertags: libfldep

libmatheval will soon fail to build from source, beause flex drops
its dependency on libfl-dev. Since libmatheval uses parts of
libfl-dev (e.g. libl.a, libfl.a or FlexLexer.h), it should add
libfl-dev to its Build-Depends. This change was previously
announced[1] to debian-devel in accordance with DevRef 7.1.1. Please
add the missing dependency.

Helmut

[1] https://lists.debian.org/debian-devel/2016/03/msg00162.html