Update m4/acx_check_strptr_convert.m4.

The previous code resulted in the following error when compiled with
GCC 6.4.0 and FCFLAGS=-std=-f2008:

TS 29113/TS 18508: Noninteroperable array FPTR at (1) to C_F_POINTER: Type shall have a character length of 1

Moreover, the code did not really reflect how the string manipulation
was implemented in src/mo_cdi.f90. The updated version is supposed to
be closer to what is required in the real code.

m4/acx_check_strptr_convert.m4

@@ -66,26 +66,24 @@ contains
       end function strlen
     end interface
     type(c_ptr) :: cptr
-    character(len=:, kind=c_char), pointer :: errstr
+    character(kind=c_char), dimension(:), pointer :: buf
+    character(len=:), pointer :: errstr
+    integer :: i, rv_shape(1)
 
     cptr = strerror(int(errno, c_int))
-    errstr => c2f_string(cptr, int(strlen(cptr)))
+    rv_shape(1) = int(strlen(cptr))
+    call c_f_pointer(cptr, buf, rv_shape)
+    allocate(character(rv_shape(1)) :: errstr)
+    do i = 1, rv_shape(1)
+      errstr(i:i) = buf(i)
+    end do
   end function errstr
-
-  function c2f_string(s, slen)
-    type(c_ptr), intent(in) :: s
-    integer, intent(in) :: slen
-    CHARACTER(len=slen, kind=c_char), POINTER :: c2f_string
-    c2f_string => NULL()
-    call c_f_pointer(s, c2f_string)
-  end function c2f_string
 end module conftest_mod
 
 program conftest
-  use iso_c_binding
   use conftest_mod, only: errstr
   implicit none
-  character(kind=c_char, len=:), pointer :: msg
+  character(len=:), pointer :: msg
   msg => errstr(42)
   write (0, '(a)') msg
 end program conftest])],