Bitcoin-Qt: expand GCC Stack-Smashing Protector usage (non Windows)

This patch currently excludes our Windows executable!

- change to use -fstack-protector-all
- the minimum size of buffers (i.e. arrays) that will receive stack
  smashing protection was changed to 1 byte (GCC default: 8)
- warn about functions that will not be protected against stack
  smashing by adding -Wstack-protector

bitcoin-qt.pro

@@ -30,6 +30,13 @@ contains(RELEASE, 1) {
     }
 }
 
+!win32 {
+# for extra security against potential buffer overflows: enable GCCs Stack Smashing Protection
+QMAKE_CXXFLAGS *= -fstack-protector-all --param ssp-buffer-size=1
+QMAKE_LFLAGS *= -fstack-protector-all --param ssp-buffer-size=1
+# We need to exclude this for Windows cross compile with MinGW 4.2.x, as it will result in a non-working executable!
+# This can be enabled for Windows, when we switch to MinGW >= 4.4.x.
+}
 # for extra security on Windows: enable ASLR and DEP via GCC linker flags
 win32:QMAKE_LFLAGS *= -Wl,--dynamicbase -Wl,--nxcompat
 
@@ -82,12 +89,6 @@ contains(BITCOIN_NEED_QT_PLUGINS, 1) {
     QTPLUGIN += qcncodecs qjpcodecs qtwcodecs qkrcodecs qtaccessiblewidgets
 }
 
-!windows {
-    # for extra security against potential buffer overflows
-    QMAKE_CXXFLAGS += -fstack-protector
-    QMAKE_LFLAGS += -fstack-protector
-    # do not enable this on windows cross compile with mingw 4.2.x, as it will result in a non-working executable!
-}
 
 # regenerate src/build.h
 !windows|contains(USE_BUILD_INFO, 1) {
@@ -99,7 +100,7 @@ contains(BITCOIN_NEED_QT_PLUGINS, 1) {
     DEFINES += HAVE_BUILD_INFO
 }
 
-QMAKE_CXXFLAGS_WARN_ON = -fdiagnostics-show-option -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter
+QMAKE_CXXFLAGS_WARN_ON = -fdiagnostics-show-option -Wall -Wextra -Wformat -Wformat-security -Wno-unused-parameter -Wstack-protector
 
 # Input
 DEPENDPATH += src src/json src/qt