Averted the relocation bug.

According to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70936, GCC's '--with-gxx-include-dir' configure option is causing a relocation bug, We'd better remove it.
The newly added standard-conforming <cstdlib> and <cmath> C++ headers since GCC 6 suffer from this bug. A relocated GCC will complain about inability to find <stdlib.h> and <math.h>, respectively.
In addition, removal of this option changes the paths where GCC searches for libstdc++ headers. We make GCC search for them in the default directories by removing '--enable-version-specific-runtime-libs' as well.

mingw-w64-gcc-git/PKGBUILD

@@ -140,7 +140,6 @@ build() {
     --target=${MINGW_CHOST} \
     --with-native-system-header-dir=${MINGW_PREFIX}/${MINGW_CHOST}/include \
     --libexecdir=${MINGW_PREFIX}/lib \
-    --with-gxx-include-dir=${MINGW_PREFIX}/include/c++/${_realpkgver} \
     --enable-bootstrap \
     --with-arch=${_arch} \
     --with-tune=generic \
@@ -153,7 +152,6 @@ build() {
     --enable-libstdcxx-time=yes \
     --disable-libstdcxx-pch \
     --disable-libstdcxx-debug \
-    --enable-version-specific-runtime-libs \
     --disable-isl-version-check \
     --enable-lto \
     --enable-libgomp \
@@ -173,6 +171,8 @@ build() {
     ${_conf}
     #--enable-libitm
     #--enable-objc-gc
+    #--with-gxx-include-dir=${MINGW_PREFIX}/include/c++/${_realpkgver} \
+    #--enable-version-specific-runtime-libs \
 
   # While we're debugging -fopenmp problems at least.
   # .. we may as well not strip anything.