Merge bitcoin/bitcoin#29195: build: Fix -Xclang -internal-isystem option

d742be3 ci: Switch native macOS CI job to Xcode 15.0 (Hennadii Stepanov)
8decc5c build: Fix `-Xclang -internal-isystem` option (Hennadii Stepanov)

Pull request description:

  This PR:
  - addresses bitcoin/bitcoin#29165 (comment)
  - fixes bitcoin/bitcoin#29174

ACKs for top commit:
  fanquake:
    ACK d742be3. The same as what was done in #27328.

Tree-SHA512: 4788a0511e9fac638edab8e4f7ec62c5e08aeb07e518ab62fd53074ab3dd4eca1f62dc17c2af2b535bad12e77a7437e5c1c714cd03ce711e5d5e5c87d4620358

Author: fanquake

.github/workflows/ci.yml

@@ -89,7 +89,9 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Clang version
-        run: clang --version
+        run: |
+          sudo xcode-select --switch /Applications/Xcode_15.0.app
+          clang --version
 
       - name: Install Homebrew packages
         env:

configure.ac

@@ -740,7 +740,7 @@ case $host in
          dnl option to system-ify all /usr/local/include paths without adding it to the list
          dnl of search paths in case it's not already there.
          if test "$suppress_external_warnings" != "no"; then
-           AX_CHECK_PREPROC_FLAG([-Xclang -internal-isystem/usr/local/include], [CORE_CPPFLAGS="$CORE_CPPFLAGS -Xclang -internal-isystem/usr/local/include"], [], [$CXXFLAG_WERROR])
+           AX_CHECK_PREPROC_FLAG([-Xclang -internal-isystem -Xclang /usr/local/include/], [CORE_CPPFLAGS="$CORE_CPPFLAGS -Xclang -internal-isystem -Xclang /usr/local/include/"], [], [$CXXFLAG_WERROR])
          fi
 
          if test "$use_bdb" != "no" && $BREW list --versions berkeley-db@4 >/dev/null && test "$BDB_CFLAGS" = "" && test "$BDB_LIBS" = ""; then