enforce GCC5 C++11 ABI and usage

The library(s) make an API break anyhow, so lets ensure we use gcc5 for
this break and enable c++11 as standard as gcc6 will use it as default
and should provide some API parts for c++11 – beside that it can't hurt
to use c++11 itself. We just have to keep our headers c++03 compatible
to not enforce a standrd bump in our reverse dependencies.

buildlib/environment.mak.in

@@ -9,7 +9,7 @@ PACKAGE_MAIL = @PACKAGE_MAIL@
 CC = @CC@
 CPPFLAGS+= @CPPFLAGS@ @DEFS@ -D_REENTRANT -D_FORTIFY_SOURCE=2
 CXX = @CXX@
-CXXFLAGS+= @CXXFLAGS@ -Wall -Wextra
+CXXFLAGS+= @CXXFLAGS@ -std=c++11 -Wall -Wextra
 CXXFLAGS+= -Wcast-align -Wlogical-op -Wredundant-decls -Wmissing-declarations -Wunsafe-loop-optimizations
 CXXFLAGS+= -Wctor-dtor-privacy -Wdisabled-optimization -Winit-self -Wmissing-include-dirs -Wnoexcept -Wsign-promo -Wundef
 # suggests methods which already have such an attribute

debian/control

@@ -9,7 +9,8 @@ Build-Depends: dpkg-dev (>= 1.15.8), debhelper (>= 8.1.3~), libdb-dev,
  gettext (>= 0.12), libcurl4-gnutls-dev (>= 7.19.4~),
  zlib1g-dev, libbz2-dev, liblzma-dev,
  xsltproc, docbook-xsl, docbook-xml, po4a (>= 0.34-2),
- autotools-dev, autoconf, automake, libgtest-dev
+ autotools-dev, autoconf, automake, libgtest-dev,
+ g++-5 (>= 5.1.1-20)
 Build-Depends-Indep: doxygen, w3m, graphviz
 Build-Conflicts: autoconf2.13, automake1.4
 Vcs-Git: git://anonscm.debian.org/apt/apt.git