[m4] change metis check such that it also support libscotchmetis

we now check for METIS_PartGraphKway, which a symbol we are actually using and which
is in the implementation of scotch (see FS#1370)
[m4] check for parmetis.h in different places

we are now checking for different locations of parmetis.h. This makes it possible us the
scotch parmetis implementation (see FS#1370).