fix(compiler_test): revert to standard conformance

This commit effectively reverts commit eca3e6b, which relied
on labeling a impure external procedure as pure via an interface.

Author: rouson

test/compiler_test.f90

@@ -2,6 +2,7 @@ module compiler_test
   !! Test compiler conformance with each scenario in which the Fortran 2018
   !! standard mandates type finalization.
   use veggies, only: result_t, test_item_t, describe, it, assert_equals, assert_that
+  use iso_fortran_env, only : compiler_version
   implicit none
 
   private
@@ -201,17 +202,34 @@ function check_specification_expression() result(result_)
     type(result_t) result_
     integer exit_status
     logical error_termination_occurred
-  
+
     call execute_command_line( &
-      command = "fpm run --example specification_expression_finalization > /dev/null 2>&1", &
+      command = "fpm run --example specification_expression_finalization "// fpm_compiler_arguments() //" > /dev/null 2>&1", &
       wait = .true., &
       exitstat = exit_status &
     )
     error_termination_occurred = exit_status /=0
     result_ = assert_that(error_termination_occurred)
 
+  contains
+
+    pure function fpm_compiler_arguments() result(args)
+      character(len=:), allocatable :: args
+
+      associate(compiler_identity=>compiler_version())
+        if (scan(compiler_identity, "GCC ")==1) then
+          args = " "
+        else if (scan(compiler_identity, "NAG Fortran ")==1) then
+          args = "--compiler nagfor --flag -fpp"
+        else
+          error stop "----> Unrecognized compiler_version() in function fpm_compiler_arguments. <----"
+        end if
+      end associate
+    end function
+
   end function
 
+
   function check_intent_out_finalization() result(result_)
     !! Test conformance with Fortran 2018 standard clause 7.5.6.3, paragraph 7:
     !! "nonpointer, nonallocatable, INTENT (OUT) dummy argument"