Link against QMAKE_LIBS_EXECINFO when using backtrace(3).

Add a new mkspec variable, QMAKE_LIBS_EXECINFO, for platforms where
backtrace(3), backtrace_symbols(3) and others are not in libc, but
rather in a separate library -- on the BSDs, this is libexecinfo.

Use it in corelib/global/global.pri so that libqt5core links against it
and has the proper dependency when necessary.

Change-Id: I62ac36c9b3ba7ab0719420cb795087d43ec138a4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

mkspecs/freebsd-g++/qmake.conf

@@ -17,6 +17,7 @@ QMAKE_LFLAGS_THREAD     = -pthread
 
 QMAKE_LIBS              =
 QMAKE_LIBS_DYNLOAD      =
+QMAKE_LIBS_EXECINFO     = -lexecinfo
 QMAKE_LIBS_X11          = -lXext -lX11 -lm
 QMAKE_LIBS_OPENGL       = -lGL
 QMAKE_LIBS_THREAD       =

mkspecs/freebsd-g++46/qmake.conf

@@ -17,6 +17,7 @@ QMAKE_LFLAGS_THREAD     = -pthread
 
 QMAKE_LIBS              =
 QMAKE_LIBS_DYNLOAD      =
+QMAKE_LIBS_EXECINFO     = -lexecinfo
 QMAKE_LIBS_X11          = -lXext -lX11 -lm
 QMAKE_LIBS_OPENGL       = -lGL
 QMAKE_LIBS_THREAD       =

mkspecs/unsupported/freebsd-clang/qmake.conf

@@ -18,6 +18,7 @@ QMAKE_LFLAGS_THREAD     = -pthread
 
 QMAKE_LIBS              =
 QMAKE_LIBS_DYNLOAD      =
+QMAKE_LIBS_EXECINFO     = -lexecinfo
 QMAKE_LIBS_X11          = -lXext -lX11 -lm
 QMAKE_LIBS_OPENGL       = -lGL
 QMAKE_LIBS_THREAD       =

src/corelib/global/global.pri

@@ -35,6 +35,9 @@ INCLUDEPATH += $$QT_BUILD_TREE/src/corelib/global
 # Only used on platforms with CONFIG += precompile_header
 PRECOMPILED_HEADER = global/qt_pch.h
 
+# qlogging.cpp uses backtrace(3), which is in a separate library on the BSDs.
+LIBS_PRIVATE += $$QMAKE_LIBS_EXECINFO
+
 linux*:!cross_compile:!static:!*-armcc* {
    QMAKE_LFLAGS += -Wl,-e,qt_core_boilerplate
    prog=$$quote(if (/program interpreter: (.*)]/) { print $1; })