Fortran: Use explicit type in Fortran 90 check

In CMakeTestFortranCompiler we build a test program using a Fortran 90
construct to check whether the compiler supports the language.  Some
compilers have options to require explicit variable types.  Fix the test
program to use an explicit variable type so it passes under such a
configuration.

Suggested-by: Neil Carlson <neil.n.carlson@gmail.com>
Merge topic 'Fortran-explicit-types' into next

7b5bc2c Fortran: Use explicit type in Fortran 90 check