configure: Detect compiler support for __attribute__((alias))

Such support is present almost everywhere, except for Xcode 9.
It is added in Xcode 10, but travis uses xcode9 by default,
so we should support it for a while yet.
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

configure

@@ -5518,6 +5518,21 @@ if compile_prog "" "" ; then
   vector16=yes
 fi
 
+########################################
+# See if __attribute__((alias)) is supported.
+# This false for Xcode 9, but has been remedied for Xcode 10.
+# Unfortunately, travis uses Xcode 9 by default.
+
+attralias=no
+cat > $TMPC << EOF
+int x = 1;
+extern const int y __attribute__((alias("x")));
+int main(void) { return 0; }
+EOF
+if compile_prog "" "" ; then
+    attralias=yes
+fi
+
 ########################################
 # check if getauxval is available.
 
@@ -7083,6 +7098,10 @@ if test "$vector16" = "yes" ; then
   echo "CONFIG_VECTOR16=y" >> $config_host_mak
 fi
 
+if test "$attralias" = "yes" ; then
+  echo "CONFIG_ATTRIBUTE_ALIAS=y" >> $config_host_mak
+fi
+
 if test "$getauxval" = "yes" ; then
   echo "CONFIG_GETAUXVAL=y" >> $config_host_mak
 fi