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ARROW-4667: [C++] Suppress unused function warnings with MinGW

Messages:

    In file included from C:/projects/arrow/cpp/src/arrow/io/file.cc:25:
    C:/projects/arrow/cpp/src/arrow/io/mman.h:179:12: warning: 'int munlock(const void*, size_t)' defined but not used [-Wunused-function]
     static int munlock(const void* addr, size_t len) {
                ^~~~~~~
    C:/projects/arrow/cpp/src/arrow/io/mman.h:171:12: warning: 'int mlock(const void*, size_t)' defined but not used [-Wunused-function]
     static int mlock(const void* addr, size_t len) {
                ^~~~~
    C:/projects/arrow/cpp/src/arrow/io/mman.h:163:12: warning: 'int msync(void*, size_t, int)' defined but not used [-Wunused-function]
     static int msync(void* addr, size_t len, int flags) {
                ^~~~~
    C:/projects/arrow/cpp/src/arrow/io/mman.h:152:12: warning: 'int mprotect(void*, size_t, int)' defined but not used [-Wunused-function]
     static int mprotect(void* addr, size_t len, int prot) {
                ^~~~~~~~

Author: Kouhei Sutou <kou@clear-code.com>

Closes #3738 from kou/cpp-suppress-warning-with-mingw and squashes the following commits:

7cf44c7f <Kouhei Sutou> Fix format
960278bb <Kouhei Sutou>  Suppress unused function warnings with MinGW