build: verify that the assembler can handle crc32 functions

Also, enable crc32 even if -msse4.2 wasn't added by us, as long as it works.
This allows custom flags (such as -march=native) to work as expected.

configure.ac

@@ -246,9 +246,32 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
   # Check for optional instruction set support. Enabling these does _not_ imply that all code will
   # be compiled with them, rather that specific objects/libs may use them after checking for runtime
   # compatibility.
-  AX_CHECK_COMPILE_FLAG([-msse4.2],[[enable_sse42=yes; SSE42_CXXFLAGS="-msse4.2"]],,[[$CXXFLAG_WERROR]])
+  AX_CHECK_COMPILE_FLAG([-msse4.2],[[SSE42_CXXFLAGS="-msse4.2"]],,[[$CXXFLAG_WERROR]])
 
 fi
+
+TEMP_CXXFLAGS="$CXXFLAGS"
+CXXFLAGS="$CXXFLAGS $SSE42_CXXFLAGS"
+AC_MSG_CHECKING(for assembler crc32 support)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+    #include <stdint.h>
+    #if defined(_MSC_VER)
+    #include <intrin.h>
+    #elif defined(__GNUC__) && defined(__SSE4_2__)
+    #include <nmmintrin.h>
+    #endif
+  ]],[[
+    uint64_t l = 0;
+    l = _mm_crc32_u8(l, 0);
+    l = _mm_crc32_u32(l, 0);
+    l = _mm_crc32_u64(l, 0);
+    return l;
+  ]])],
+ [ AC_MSG_RESULT(yes); enable_hwcrc32=yes],
+ [ AC_MSG_RESULT(no)]
+)
+CXXFLAGS="$TEMP_CXXFLAGS"
+
 CPPFLAGS="$CPPFLAGS -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS"
 
 AC_ARG_WITH([utils],
@@ -1132,7 +1155,7 @@ AM_CONDITIONAL([USE_QRCODE], [test x$use_qr = xyes])
 AM_CONDITIONAL([USE_LCOV],[test x$use_lcov = xyes])
 AM_CONDITIONAL([GLIBC_BACK_COMPAT],[test x$use_glibc_compat = xyes])
 AM_CONDITIONAL([HARDEN],[test x$use_hardening = xyes])
-AM_CONDITIONAL([ENABLE_SSE42],[test x$enable_sse42 = xyes])
+AM_CONDITIONAL([ENABLE_HWCRC32],[test x$enable_hwcrc32 = xyes])
 
 AC_DEFINE(CLIENT_VERSION_MAJOR, _CLIENT_VERSION_MAJOR, [Major version])
 AC_DEFINE(CLIENT_VERSION_MINOR, _CLIENT_VERSION_MINOR, [Minor version])