Add a linker version script to Qt libraries

This linker script is only enabled for systems with GCC or GCC-like
compilers, though technically it should work on the BSDs too (will
enable after testing). For regular modules, this declares one ELF
version "Qt_5" and places all QtCore symbols inside, then it declares
unused ELF versions "Qt_5.x" for each older minor release. For modules
declared "internal_module", all symbols are placed in version
Qt_5_PRIVATE_API.

The big advantage of an ELF version is that, when we do Qt 6, both
versions of QtCore could be loaded in memory without conflicts and all
symbols would be resolved to the correct library. No module can talk to
both at the same time, but this avoids mistakes of loading them
indirectly by plugins.

The extra Qt_5.x versions will be used in the next commit.

Change-Id: I049a653beeb5454c9539ffff13e3fe6f050fdf31
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

mkspecs/common/gcc-base-unix.conf

@@ -22,3 +22,4 @@ QMAKE_LFLAGS_USE_GOLD   = -fuse-ld=gold
 # -Bsymbolic-functions (ld) support
 QMAKE_LFLAGS_BSYMBOLIC_FUNC = -Wl,-Bsymbolic-functions
 QMAKE_LFLAGS_DYNAMIC_LIST   = -Wl,--dynamic-list,
+QMAKE_LFLAGS_VERSION_SCRIPT = -Wl,--version-script,

mkspecs/common/qcc-base-qnx.conf

@@ -19,6 +19,7 @@ QMAKE_LFLAGS_RPATHLINK  = -Wl,-rpath-link,
 # -Bsymbolic-functions (ld) support
 QMAKE_LFLAGS_BSYMBOLIC_FUNC = -Wl,-Bsymbolic-functions
 QMAKE_LFLAGS_DYNAMIC_LIST   = -Wl,--dynamic-list,
+QMAKE_LFLAGS_VERSION_SCRIPT = -Wl,--version-script,
 
 # Generic options for all BlackBerry/QNX qcc mkspecs
 QMAKE_CFLAGS_THREAD     = -D_REENTRANT

mkspecs/features/qt_module.prf

@@ -185,6 +185,28 @@ equals(QT_ARCH, i386):contains(QT_CPU_FEATURES.$$QT_ARCH, sse2):compiler_support
     QMAKE_CXXFLAGS += -mfpmath=sse
 }
 
+unix:!isEmpty(QMAKE_LFLAGS_VERSION_SCRIPT):!no_linker_version_script:!static {
+    verscript = $$OUT_PWD/$${TARGET}.version
+    QMAKE_LFLAGS += $${QMAKE_LFLAGS_VERSION_SCRIPT}$$verscript
+
+    internal_module {
+        verscript_content = "Qt_$${QT_MAJOR_VERSION}_PRIVATE_API { *; };"
+    } else {
+        current = Qt_$$QT_MAJOR_VERSION
+        verscript_content = "$$current { *; };"
+        for(i, 0..$$section(VERSION, ., 1, 1)) {
+            previous = $$current
+            current = Qt_$${QT_MAJOR_VERSION}.$$i
+            verscript_content += "$$current {} $$previous;"
+        }
+    }
+    write_file($$verscript, verscript_content)|error("Aborting.")
+    unset(current)
+    unset(previous)
+    unset(verscript)
+    unset(verscript_content)
+}
+
 android: CONFIG += qt_android_deps
 
 #install directives

mkspecs/linux-icc/qmake.conf

@@ -104,6 +104,7 @@ QMAKE_CXXFLAGS_PRECOMPILE = -c -pch-create ${QMAKE_PCH_OUTPUT} -include ${QMAKE_
 # -Bsymbolic-functions (ld) support
 QMAKE_LFLAGS_BSYMBOLIC_FUNC = -Wl,-Bsymbolic-functions
 QMAKE_LFLAGS_DYNAMIC_LIST = -Wl,--dynamic-list,
+QMAKE_LFLAGS_VERSION_SCRIPT = -Wl,--version-script,
 
 # Symbol visibility control
 QMAKE_CFLAGS_HIDESYMS   += -fvisibility=hidden

mkspecs/unsupported/linux-host-g++/qmake.conf

@@ -71,6 +71,7 @@ QMAKE_PCH_OUTPUT_EXT    = .gch
 # -Bsymbolic-functions (ld) support
 QMAKE_LFLAGS_BSYMBOLIC_FUNC = -Wl,-Bsymbolic-functions
 QMAKE_LFLAGS_DYNAMIC_LIST = -Wl,--dynamic-list,
+QMAKE_LFLAGS_VERSION_SCRIPT = -Wl,--version-script,
 
 #
 # qmake configuration for common linux