Fix bug with variables

Thanks to @abelsiqueira to report this problem.

Steps to Reproduce
------------------

~~~
$ echo $FC

$ echo $CC

$ ./configure
...
$ make
...
~~~

Actual Results
--------------

Build process fail due

~~~
/bin/sh: f77: command not found
~~~

Expected Results
----------------

Build without problem.

Explanation
-----------

[GNU Make](http://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html)
default value for `FC` is `f77`. If `FC` is unset and

~~~
FC?=some_fortran_compilr
~~~

used in `Makefile` GNU Make will assign `f77` for `FC`.

Solution
--------

Use

~~~
FC=some_fortran_compilr
~~~
Update version of sifdecode