Merge #584: configure: Use CFLAGS_FOR_BUILD when checking native compiler

Merge #584: configure: Use CFLAGS_FOR_BUILD when checking native compiler

a34bcaa Actually pass CFLAGS_FOR_BUILD and LDFLAGS_FOR_BUILD to linker (Tim Ruffing)
2d5f4ce configure: Use CFLAGS_FOR_BUILD when checking native compiler (Tim Ruffing)

Pull request description:

  This fixes a bug where configure would fail or disable static
  ecmult tables because it wrongly checks the native compiler using
  the target CFLAGS (instead of the native CFLAGS_FOR_BUILD).

  Moreover, this commit adds tests to figure out whether the native
  compiler supports the warning flags passed during the build, and it
  contains a few minor improvements to the code that checks the native
  compiler.

Tree-SHA512: 31a92a5516cf2f9801c918edfba0059aa4f8549b0c1de94fc166b5e92ad1868a480c48cdc5ff62679ba20e26f4a0e2948c71fd2b3e80766673d2bf7512da3875