Move checkFmt to top of file to avoid compiler bug

Some versions of gfortran ICE with the function in
the body of the file. Defining it first seems to be
fine and should be valid Fortran 2008.
See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85138
and #57

fsys/fox_m_fsys_format.F90

@@ -123,6 +123,30 @@ module fox_m_fsys_format
 contains
 
 #ifndef DUMMYLIB
+  ! NB: we need checkFmt at the top of the file 
+  ! to work around a bug in some Gfortran versions
+  ! see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85138
+  pure function checkFmt(fmt) result(good)
+    character(len=*), intent(in) :: fmt
+    logical :: good
+
+    ! should be ([rs]\d*)?
+
+    if (len(fmt) > 0) then
+      if (fmt(1:1) == "r" .or. fmt(1:1) == "s") then
+        if (len(fmt) > 1) then
+          good = (verify(fmt(2:), digit) == 0)
+        else
+          good = .true.
+        endif
+      else
+        good = .false.
+      endif
+    else
+      good = .true.
+    endif
+  end function checkFmt
+
   ! NB: The len generic module procedure is used in
   !     many initialisation statments (to set the
   !     length of the output string needed for the
@@ -2216,29 +2240,6 @@ pure function str_complex_dp_matrix(ca) result(s)
 #endif
   end function str_complex_dp_matrix
 
-#ifndef DUMMYLIB
-  pure function checkFmt(fmt) result(good)
-    character(len=*), intent(in) :: fmt
-    logical :: good
-
-    ! should be ([rs]\d*)?
-
-    if (len(fmt) > 0) then
-      if (fmt(1:1) == "r" .or. fmt(1:1) == "s") then
-        if (len(fmt) > 1) then
-          good = (verify(fmt(2:), digit) == 0)
-        else
-          good = .true.
-        endif
-      else
-        good = .false.
-      endif
-    else
-      good = .true.
-    endif
-  end function checkFmt
-#endif
-
   pure function concat_str_int(s1, s2) result(s3)
     character(len=*), intent(in) :: s1
     integer, intent(in) :: s2