Derived types in GP module require gfortran 4.7

[j-faria]

From the GCC 4.7 release notes https://gcc.gnu.org/gcc-4.7/changes.html

Fortran 2003:
Generic interface names which have the same name as derived types are 
now supported, which allows to write constructor functions. Note that Fortran 
does not support static constructor functions; only default initialization or an 
explicit structure-constructor initialization are available.