configure: Fix for clang++-8 and newer

AX_CHECK_COMPILE_FLAG fails if it is used with -Werror and the compiler
raises error -Wextra-semi-stmt:

    configure:4224: checking whether C++ compiler accepts -mavx
    configure:4243: clang++-8 -c -g -O2 -Wall -Wextra -Wpedantic -Weverything -Wno-c++98-compat -Wno-c++98-compat-pedantic -march=native -Werror -Wno-unused-macros -mavx  conftest.cpp >&5
    conftest.cpp:20:3: error: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Werror,-Wextra-semi-stmt]
      ;
      ^
    1 error generated.

Add -Wno-extra-semi-stmt to disable those errors if possible.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

configure.ac

@@ -111,9 +111,12 @@ case "${host_os}" in
         ;;
 esac
 
+WERROR=-Werror
+# The test code used by AX_CHECK_COMPILE_FLAG uses an empty statement.
+# clang++-8 can emit a warning for that which must be disabled.
+AX_CHECK_COMPILE_FLAG([-Wno-extra-semi-stmt], [WERROR="$WERROR -Wno-extra-semi-stmt"])
 # The test code used by AX_CHECK_COMPILE_FLAG uses macros which are unused.
 # That must not break the check, so disable a related warning if necessary.
-WERROR=-Werror
 AX_CHECK_COMPILE_FLAG([-Wno-unused-macros], [WERROR="$WERROR -Wno-unused-macros"])
 
 ## Checks for supported compiler options.