macos-setup.sh: Set --build for x86_64 GMP build.

Without specifying a CPU family, the GMP configure script configures for the
CPU type of the build host.  This can break things for users with older Intel
CPUs when we have a builder with a newer CPU.

For x86_64, set "nehalem" as the CPU architecture, since that's the oldest CPU
architecture supported by the oldest version of macOS we presently support.
Since Apple is no longer using Intel CPUs, this can probably remain as-is until
we stop building for Intel Mac.

Tidy up a few comments.

Fixes #20348.

tools/macos-setup.sh

@@ -121,7 +121,7 @@ PCRE2_VERSION=10.44
 #
 # To override the version of Qt call the script with some of the variables
 # set to the new values. Setting the variable to empty will disable building
-# the toolkit and will uninstall # any version previously installed by the
+# the toolkit and will uninstall any version previously installed by the
 # script, e.g.
 # "QT_VERSION=5.10.1 ./macos-setup.sh"
 # will build and install with QT 5.10.1.
@@ -1425,8 +1425,17 @@ install_gmp() {
         else
             LD64_FLAG=""
         fi
+        if [ "$DARWIN_PROCESSOR_ARCH" = "x86_64" ]
+        then
+            # If contemplating darwin20 or newer, refer to the last two paragraphs of
+            # https://gmplib.org/list-archives/gmp-bugs/2024-October/005539.html
+            # and either ensure that GMP is newer than 6.3.0 or run "autoreconf".
+            GMP_BUILD_OPTION="--build=nehalem-apple-darwin18"
+        else
+            GMP_BUILD_OPTION=""
+        fi
         CFLAGS="$CFLAGS $VERSION_MIN_FLAGS $SDKFLAGS" CXXFLAGS="$CXXFLAGS $VERSION_MIN_FLAGS $SDKFLAGS" LDFLAGS="$LDFLAGS $VERSION_MIN_FLAGS $SDKFLAGS $LD64_FLAG" \
-            ./configure "${CONFIGURE_OPTS[@]}" --enable-fat
+            ./configure "${CONFIGURE_OPTS[@]}" --enable-fat "$GMP_BUILD_OPTION"
         make "${MAKE_BUILD_OPTS[@]}"
         $DO_MAKE_INSTALL
         cd ..
@@ -4112,7 +4121,7 @@ fi
 # You need Xcode or the command-line tools installed to get the compilers (xcrun checks both).
 #
  if [ ! -x /usr/bin/xcrun ]; then
-    echo "Please install Xcode (app or command line) first (should be available on DVD or from the Mac App Store)."
+    echo "Please install Xcode (app or command line) first (should be available from the Mac App Store)."
     exit 1
 fi
 
@@ -4132,7 +4141,7 @@ if [ "$QT_VERSION" ]; then
     elif qmake --version >/dev/null 2>&1; then
         :
     else
-        echo "Please install Xcode first (should be available on DVD or from the Mac App Store)."
+        echo "Please install Xcode first (should be available from the Mac App Store)."
         echo "The command-line build tools are not sufficient to build Qt."
         echo "Alternatively build QT according to: https://gist.github.com/shoogle/750a330c851bd1a924dfe1346b0b4a08#:~:text=MacOS%2FQt%5C%20Creator-,Go%20to%20Qt%20Creator%20%3E%20Preferences%20%3E%20Build%20%26%20Run%20%3E%20Kits,for%20both%20compilers%2C%20not%20gcc%20."
         exit 1