Use GCC -mno-direct-extern-access when building an x86/x86_64 coda_qt executable

...so that it wouldn't provide and export _ZN16QCoreApplication4selfE@Qt_6 when
it would mention any use of e.g. the qApp macro, which could cause coda_qt to
terminate with a SIGSEGV at start (cf. 9c8e658
"avoid Crash on startup of CODA-Q").

See the mailing list thread starting at
<https://lists.qt-project.org/pipermail/development/2025-December/046791.html>
"[Development] QT_FEATURE_reduce_relocations breaking executables that mention
qApp?" for details.

(Not entirely clear to me whether we would need -mno-direct-extern-access also
on any shared libraries (that link against Qt), if we would build any, but lets
restrict this to the CXXFLAGS of the coda_qt executable for now.)

Signed-off-by: Stephan Bergmann <[email protected]>
Change-Id: Iee5bcf0e1507692a8a4d3f2753f75c362fe9777e

Author: stbergmann

configure.ac

@@ -66,6 +66,8 @@ AC_DEFINE_UNQUOTED([COOLWSD_BUILDCONFIG],[["$ac_configure_args"]],[Options passe
 # don't include config_unused.h anywhere!
 AC_CONFIG_HEADERS([config_unused.h])
 
+AM_CONDITIONAL([X86], [test "$host_cpu" = x86 || test "$host_cpu" = x86_64])
+
 # Checks for programs.
 AC_PROG_CXX
 AC_PROG_CC

qt/Makefile.am

@@ -74,6 +74,11 @@ DBusService.moc.cpp: DBusService.hpp
 
 CLEANFILES = $(BUILT_SOURCES)
 
+coda_qt_CXXFLAGS = $(AM_CXXFLAGS)
+if X86
+coda_qt_CXXFLAGS += -mno-direct-extern-access
+endif
+
 coda_qt_SOURCES = coda-qt.cpp WebView.cpp DBusService.cpp RecentDocuments.cpp $(BUILT_SOURCES) $(cmake_list)
 
 metainfodir = $(datadir)/metainfo