package/ci: avoid tons of warnings from GCC 4.8.

By including pybind11 and Python headers as SYSTEM. Possible in pybind11
master, but not yet part of any release so I'm patching it manually.

package/ci/pybind11-system-includes.patch

@@ -0,0 +1,13 @@
+diff --git a/tools/pybind11Tools.cmake b/tools/pybind11Tools.cmake
+index 52a70c2..1e9d033 100644
+--- a/tools/pybind11Tools.cmake
++++ b/tools/pybind11Tools.cmake
+@@ -130,7 +130,7 @@ function(pybind11_add_module target_name)
+ 
+   add_library(${target_name} ${lib_type} ${exclude_from_all} ${ARG_UNPARSED_ARGUMENTS})
+ 
+-  target_include_directories(${target_name}
++  target_include_directories(${target_name} SYSTEM
+     PRIVATE ${PYBIND11_INCLUDE_DIR}  # from project CMakeLists.txt
+     PRIVATE ${pybind11_INCLUDE_DIR}  # from pybind11Config
+     PRIVATE ${PYTHON_INCLUDE_DIRS})

package/ci/setup-pybind11.sh

@@ -0,0 +1,20 @@
+set -e
+
+wget --no-clobber https://github.com/pybind/pybind11/archive/v2.2.4.tar.gz && tar -xzf v2.2.4.tar.gz
+
+cd pybind11-2.2.4
+
+# Patch the CMake macro to add include paths as SYSTEM because otherwise GCC
+# 4.8 gets very loud. This is already done in
+# https://github.com/pybind/pybind11/pull/1416 but it's opt-in and not part of
+# any release yet. I don't want to force users to use pybind11 git, so apply
+# a patch manually.
+patch -p1 < $TRAVIS_BUILD_DIR/package/ci/pybind11-system-includes.patch
+
+mkdir -p build && cd build
+cmake .. \
+    -DCMAKE_INSTALL_PREFIX=$HOME/pybind11 \
+    -DPYBIND11_PYTHON_VERSION=3.6 \
+    -DPYBIND11_TEST=OFF \
+    -G Ninja
+ninja install

package/ci/travis.yml

@@ -48,7 +48,7 @@ install:
 - if [ "$TRAVIS_OS_NAME" == "linux" ] || [ "$TRAVIS_OS_NAME" == "osx" ]; then pip3 install coverage codecov; fi
 
 # pybind11 (cached)
-- if [ "$TRAVIS_OS_NAME" == "linux" ] && [ ! -e "$HOME/pybind11/bin" ]; then wget -nc https://github.com/pybind/pybind11/archive/v2.2.4.tar.gz && tar -xzf v2.2.4.tar.gz && cd pybind11-2.2.4 && mkdir -p build && cd build && cmake .. -DCMAKE_INSTALL_PREFIX=$HOME/pybind11 -DPYBIND11_PYTHON_VERSION=3.6 -DPYBIND11_TEST=OFF -G Ninja && ninja install && cd $TRAVIS_BUILD_DIR; fi
+- if [ "$TRAVIS_OS_NAME" == "linux" ] && [ ! -e "$HOME/pybind11/bin" ]; then $TRAVIS_BUILD_DIR/package/ci/setup-pybind11.sh; fi
 - if [ "$TRAVIS_OS_NAME" == "osx" ]; then HOMEBREW_NO_AUTO_UPDATE=1 brew install pybind11; fi
 
 script: