Add -march=core-avx2 for AMD CPUs compiled using ICC

Speeds up self-gravity on my EAGLE_50 tests using Intel/2018 plus:

./configure --with-tbbmalloc --with-parmetis --enable-debug

full steps go from 155 down to 105 (x2 in log message included some I/O). These are MPI runs on a single node using all the cores (4x16=64).

Only known to work for EPYC at Durham, but patterns for other AMDs that claim AVX2 support are included. See:

https://www.amd.com/system/files/documents/compiler-options-guide-amd-epyc-7xx1-series-processors.pdf

m4/ax_cc_maxopt.m4

@@ -133,6 +133,14 @@ if test "$ac_test_CFLAGS" != "set"; then
 		    *000?f[[346]]?:*:*:*|?f[[346]]?:*:*:*|f[[346]]?:*:*:*) icc_flags="-xSSE3 -xP -xO -xN -xW -xK" ;;
 		    *00??f??:*:*:*|??f??:*:*:*|?f??:*:*:*|f??:*:*:*) icc_flags="-xSSE2 -xN -xW -xK" ;;
                   esac ;;
+                *:68747541:444d4163:69746e65) # AMDs with AVX2 support.
+                  case $ax_cv_gcc_x86_cpuid_1 in
+                    *061?f??:*:*:*|61?f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                    *06??f??:*:*:*|6??f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                    *070?f??:*:*:*|70?f??:*:*:*) icc_flags="-march=core-avx2" ;;
+                                   83?f??:*:*:*) icc_flags="-march=core-avx2"
+                                                 CFLAGS="$CFLAGS -fma -ftz -fomit-frame-pointer";; # EPYC
+                  esac ;;
               esac ;;
           esac
           if test "x$icc_flags" != x; then