Compiler warnings with both Givaro and FLINT 3.1.0 (longlong.h v reclonglong.h)

[d-torrance]

When compiling a simple program with both Givaro and FLINT headers after the recent FLINT 3.1.0 release, I get the following compiler warnings if the Givaro headers are included before the FLINT headers (but not the other way around):

$ g++ foo.cpp
In file included from foo.cpp:7:
flint/longlong.h:74: warning: "__ll_B" redefined
   74 | #define __ll_B ((ulong) 1 << (FLINT_BITS / 2))
      | 
In file included from foo.cpp:5:
/usr/include/recint/reclonglong.h:46: note: this is the location of the previous definition
   46 | #define __ll_B ((UWtype) 1 << (W_TYPE_SIZE / 2))
      | 
flint/longlong.h:75: warning: "__ll_lowpart" redefined
   75 | #define __ll_lowpart(t) ((ulong) (t) & (__ll_B - 1))
      | 
/usr/include/recint/reclonglong.h:47: note: this is the location of the previous definition
   47 | #define __ll_lowpart(t) ((UWtype) (t) & (__ll_B - 1))
      | 
flint/longlong.h:76: warning: "__ll_highpart" redefined
   76 | #define __ll_highpart(t) ((ulong) (t) >> (FLINT_BITS / 2))
      | 
/usr/include/recint/reclonglong.h:48: note: this is the location of the previous definition
   48 | #define __ll_highpart(t) ((UWtype) (t) >> (W_TYPE_SIZE / 2))
      | 

There have been similar issues with these two header files in the past -- see #99 and #137.