Merged in new upstream version of the maxopt autotools macros that fixes the…

Merged in new upstream version of the maxopt autotools macros that fixes the problem or erasing the CFLAGS.

See http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=commitdiff;h=73ee1b396c21062ee8eeb8721ba5323322110fb5 for the upstream commit.

This allows us to remove the workaround that was built into our configure script.

m4/ax_cc_maxopt.m4

 # ===========================================================================
-#       http://www.gnu.org/software/autoconf-archive/ax_cc_maxopt.html
+#       https://www.gnu.org/software/autoconf-archive/ax_cc_maxopt.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -40,7 +40,7 @@
 #   Public License for more details.
 #
 #   You should have received a copy of the GNU General Public License along
-#   with this program. If not, see <http://www.gnu.org/licenses/>.
+#   with this program. If not, see <https://www.gnu.org/licenses/>.
 #
 #   As a special exception, the respective Autoconf Macro's copyright owner
 #   gives unlimited permission to copy, distribute and modify the configure
@@ -55,7 +55,7 @@
 #   modified version of the Autoconf Macro, you may extend this special
 #   exception to the GPL to apply to your modified version as well.
 
-#serial 16
+#serial 18
 
 AC_DEFUN([AX_CC_MAXOPT],
 [
@@ -68,19 +68,18 @@ AC_ARG_ENABLE(portable-binary, [AS_HELP_STRING([--enable-portable-binary], [disa
 
 # Try to determine "good" native compiler flags if none specified via CFLAGS
 if test "$ac_test_CFLAGS" != "set"; then
-  CFLAGS=""
   case $ax_cv_c_compiler_vendor in
-    dec) CFLAGS="-newc -w0 -O5 -ansi_alias -ansi_args -fp_reorder -tune host"
+    dec) CFLAGS="$CFLAGS -newc -w0 -O5 -ansi_alias -ansi_args -fp_reorder -tune host"
 	 if test "x$acx_maxopt_portable" = xno; then
            CFLAGS="$CFLAGS -arch host"
          fi;;
 
-    sun) CFLAGS="-native -fast -xO5 -dalign"
+    sun) CFLAGS="$CFLAGS -native -fast -xO5 -dalign"
 	 if test "x$acx_maxopt_portable" = xyes; then
 	   CFLAGS="$CFLAGS -xarch=generic"
          fi;;
 
-    hp)  CFLAGS="+Oall +Optrs_ansi +DSnative"
+    hp)  CFLAGS="$CFLAGS +Oall +Optrs_ansi +DSnative"
 	 if test "x$acx_maxopt_portable" = xyes; then
 	   CFLAGS="$CFLAGS +DAportable"
 	 fi;;
@@ -91,8 +90,8 @@ if test "$ac_test_CFLAGS" != "set"; then
            xlc_opt="-qtune=auto"
 	 fi
          AX_CHECK_COMPILE_FLAG($xlc_opt,
-		CFLAGS="-O3 -qansialias -w $xlc_opt",
-               [CFLAGS="-O3 -qansialias -w"
+		CFLAGS="$CFLAGS -O3 -qansialias -w $xlc_opt",
+               [CFLAGS="$CFLAGS -O3 -qansialias -w"
                 echo "******************************************************"
                 echo "*  You seem to have the IBM  C compiler.  It is      *"
                 echo "*  recommended for best performance that you use:    *"
@@ -105,7 +104,7 @@ if test "$ac_test_CFLAGS" != "set"; then
                 echo "******************************************************"])
          ;;
 
-    intel) CFLAGS="-O3 -ansi-alias"
+    intel) CFLAGS="$CFLAGS -O3 -ansi-alias"
 	if test "x$acx_maxopt_portable" = xno; then
 	  icc_archflag=unknown
 	  icc_flags=""
@@ -151,7 +150,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 
     clang)
      # default optimization flags for clang on all systems
-     CFLAGS="-O3 -fomit-frame-pointer"
+     CFLAGS="$CFLAGS -O3 -fomit-frame-pointer"
 
      # Always good optimisation to have
      AX_CHECK_COMPILE_FLAG(-fstrict-aliasing, CFLAGS="$CFLAGS -fstrict-aliasing")
@@ -167,7 +166,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 
     gnu)
      # default optimization flags for gcc on all systems
-     CFLAGS="-O3 -fomit-frame-pointer"
+     CFLAGS="$CFLAGS -O3 -fomit-frame-pointer"
 
      # -malign-double for x86 systems
      AX_CHECK_COMPILE_FLAG(-malign-double, CFLAGS="$CFLAGS -malign-double")
@@ -187,7 +186,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 
     microsoft)
      # default optimization flags for MSVC opt builds
-     CFLAGS="-O2"
+     CFLAGS="$CFLAGS -O2"
      ;;
   esac
 
@@ -199,7 +198,7 @@ if test "$ac_test_CFLAGS" != "set"; then
 	echo "* (otherwise, a default of CFLAGS=-O3 will be used)    *"
 	echo "********************************************************"
 	echo ""
-        CFLAGS="-O3"
+        CFLAGS="$CFLAGS -O3"
   fi
 
   AX_CHECK_COMPILE_FLAG($CFLAGS, [], [
@@ -210,7 +209,6 @@ if test "$ac_test_CFLAGS" != "set"; then
         echo "* Use ./configure CFLAGS=... to specify your own flags *"
         echo "********************************************************"
         echo ""
-        CFLAGS=""
   ])
 
 fi