make pandoc optional

We don't generate any doc if pandoc isn't available, but allow the project to
compile.
add a uniform int distribution to add-property

Sometimes, a discrete uniform distribution is interesting for properties.
This patch provides one, based on the gsl functions.
[fix] fix memory leaks and bad typing in function calls

tree-* functions don't destroy their temporary variables.
vector functions didn't support empty vectors.
lsa was bugging on architectures were sizeof(double)!=sizeof(unsigned long).

igraph_vector_t is a really annoying type, as it store everything in doubles
and we use it quite a lot for unsigned long data. Using an intermediate
variable ensure that the compiler will generate the right conversions.
add automake 1.12/libtool compat for recent systems

libtool complains if AM_PROG_AR is not in the configure, but it doesn't exists
on old systems. the m2_ifdef ensure the configure.ac is right for every
versions.
add edge-joint paths analysis

This algorithm is an approx, the analysis is NP-complete even on dags.
We make no guaranties on it.
The error checking is less than perfect too
Merge branch 'tasktree'
New error checking facility for ggen-lib

Inspired by igraph error handling, manages a stack of destructors to call when
exiting a function after errors.

Next patches will change ggen code to use this facility.
add error checking to graph generation

ggen.h gains a strerror function
builtin is modified to print message on errors.
add error checking to graph analysis

support functions also modified to make error checking work.
ggen-lib doesn't check for dag input anymore.
add error checking to graph transformation
add error reporting to all builtins
simplify core command management

the "one big function" was not really nice