fix(compiling): improve portability of builds

This makes the -march=native CFLAG optional, and disabled by default,
but still available through a new --native-march flag.

It should make builds more portable between machines, as previously it
was very common to get a CPU architecture error on launch if you moved
the build to a different machine running a different generation of a
Intel CPU.

From what I've understood, when using the -march=native CFLAG clang will
make as many optimizations possible based on the exact set of CPU
instructions available on the specific CPU it's compiling on.

In theory this leads to a more optimized build, though I haven't
personally noticed any difference. But it also leads to less portable
builds, for example builds from a Intel-based 2020 MacBook Pro just
crash with a unsupported CPU architecture error when run on a
Intel-based 2016 MacBook Pro.

README.md

@@ -74,6 +74,7 @@ Options:
         --git-sha SHA                Override detected git SHA of specified branch allowing builds of old commits
         --[no-]xwidgets              Enable/disable XWidgets if supported (default: enabled)
         --[no-]native-comp           Enable/disable native-comp (default: enabled if supported)
+        --[no-]native-march          Enable/disable -march=native CFLAG(default: disabled)
         --[no-]native-full-aot       Enable/disable NATIVE_FULL_AOT / Ahead of Time compilation (default: disabled)
         --[no-]rsvg                  Enable/disable SVG image support via librsvg (default: enabled)
         --no-titlebar                Apply no-titlebar patch (default: disabled)

build-emacs-for-macos

@@ -267,7 +267,7 @@ class Build
           "-I#{File.join(gcc_info.root_dir, 'include')}",
           "-I#{File.join(gcc_info.libgccjit_root_dir, 'include')}",
           '-O2',
-          '-march=native',
+          (options[:native_march] ? '-march=native' : nil),
           ENV['CFLAGS']
         ].compact.join(' ')
 
@@ -904,6 +904,7 @@ if __FILE__ == $PROGRAM_NAME
   cli_options = {
     work_dir: File.expand_path(__dir__),
     native_full_aot: false,
+    native_march: false,
     parallel: Etc.nprocessors,
     rsvg: true,
     xwidgets: true,
@@ -945,6 +946,12 @@ if __FILE__ == $PROGRAM_NAME
         cli_options[:native_comp] = v
       end
 
+      opts.on('--[no-]native-march',
+              'Enable/disable -march=native CFLAG' \
+              '(default: disabled)') do |v|
+        cli_options[:native_march] = v
+      end
+
       opts.on('--[no-]native-full-aot',
               'Enable/disable NATIVE_FULL_AOT / Ahead of Time compilation ' \
               '(default: disabled)') do |v|