xxx_LDFLAGS place the libraries like '-lX11' or '-lXft' at the beginning of
the linker command. Some systems were not able to handle the situation and
this lead to a lot of unresolved symbols. Reading the documentation of
automake:
... you can use LDADD to do so. This variable is used to specify
additional objects or libraries to link with; it is inappropriate for
specifying specific linker flags, you should use AM_LDFLAGS for this
purpose.
In addition we link against -lm in order to satisfy symbols refering to 'cos'
and 'sin'.
