cc/gcc: add option do disable PCH

In some cases, using Pre-Compiled Headers breaks the build.
Ass an option to disable building the PCH, as suggested by:
  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40974

config/cc/gcc.in

@@ -268,6 +268,18 @@ config CC_GCC_ENABLE_TARGET_OPTSPACE
 
       This will compile crossgcc's libs with -Os.
 
+config CC_GCC_DISABLE_PCH
+    bool
+    prompt "Do not build PCH"
+    default n
+    help
+      Say 'y' here to not use Pre-Compiled Headers in the resulting toolchain.
+      at the expense of speed when compiling C++ code.
+      
+      For some configurations (most notably canadian?), PCH are broken, and
+      need to be disabled.  Please see:
+        http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40974
+
 config CC_PKGVERSION
     string
     prompt "gcc ID string"

scripts/build/cc/gcc.sh

@@ -367,6 +367,9 @@ do_cc() {
     if [ "${CT_CC_GCC_ENABLE_TARGET_OPTSPACE}" = "y" ]; then
         extra_config+=("--enable-target-optspace")
     fi
+    if [ "${CT_CC_GCC_DISABLE_PCH}" = "y" ]; then
+        extra_config+=("--disable-libstdcxx-pch")
+    fi
 
     CT_DoLog DEBUG "Extra config passed: '${extra_config[*]}'"