Build issue on Ubuntu 16.04 due to wrong pkg-config CFLAGS in cairomm

Hi Tiago,

I have a build issue that I think is a bug. I think the flag is reset to c++11 after cairo file build.

After compiling cairo the next file fails:

CXX graph_cairo_draw.lo CXX graph_tree_cts.lo

The first error is (among many others ofc 😄 ): error: 'auto' return without trailing return type; deduced return types are a C++14 extension

================================================================================
                             CONFIGURATION SUMMARY                              
================================================================================
Using python version:   3.5.2
Python interpreter:     /usr/bin/python3.5
Installation path:      /usr/lib/python3/dist-packages/graph_tool

C++ compiler (CXX):     clang++-4.0 -std=gnu++14
C++ compiler version:   4.2.1

Prefix:                 /usr/local
Pkgconfigdir:           ${libdir}/pkgconfig

Python CPP flags:       -I/usr/include/python3.5m
Python LD flags:        -L/usr/lib -lpython3.5m
Boost CPP flags:        -pthread -I/usr/include
Boost LD flags:         -lboost_iostreams -lboost_python-py35 -lboost_regex -lboost_coroutine
Numpy CPP flags:        -I/usr/local/lib/python3.5/dist-packages/numpy/core/include
Sparsehash CPP flags:   
CGAL CPP flags:         -I/usr/include
CGAL LD flags:          -L/usr/lib -lCGAL -lCGAL_Core -lgmp -lboost_thread -lpthread
Expat CPP flags:        -I/usr/include
Expat LD flags:         -L/usr/lib -lexpat
Cairomm CPP flags:      -std=c++11 -I/usr/include/cairomm-1.0 -I/usr/lib/x86_64-linux-gnu/cairomm-1.0/include -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/sigc++-2.0 -I/usr/lib/x86_64-linux-gnu/sigc++-2.0/include
Cairomm LD flags:       -lcairomm-1.0 -lcairo -lsigc-2.0
OpenMP compiler flags:  -fopenmp
OpenMP LD flags:        -lomp 
Extra CPPFLAGS:         -DNDEBUG 
Extra CXXFLAGS:         -fopenmp -O3 -fvisibility=default -fvisibility-inlines-hidden -Wno-deprecated -Wall -Wextra -ftemplate-backtrace-limit=0 
Extra LDFLAGS:          

Using OpenMP:           yes
Using sparsehash:       yes
Using cairo:            yes
================================================================================