Fix CMake coarray introspection problem

 Fix problem wherein the change in f90 source file name extension in the
 hacked CMake module, removed in 5527264, was not ported
 forwards. Intrinsic CheckFortranSourceCompiles writes a .F file
 extension, and mpif90/gcc5.x would barf upon encountering free format
 code.

Author: zbeekman

CMakeLists.txt

@@ -88,7 +88,7 @@ else()
 endif()
 
 if(gfortran_compiler)
-  set(CMAKE_REQUIRED_FLAGS "-fcoarray=single")
+  set(CMAKE_REQUIRED_FLAGS "-fcoarray=single -ffree-form")
 endif()
 include(CheckFortranSourceCompiles)
 CHECK_Fortran_SOURCE_COMPILES("