cacheset: Fix compilation on new GCC in C++98 mode

Since gcc 4.9, the API for erase slightly changed. In
commit 3dddcdf2432e78f37c74d8c76c2c519a8d935ab2 the
existing checks for __cplusplus where changed to
check the gcc version, as the __cplusplus check
did nothing, because gcc 4.8 already provided the
standard value in there.

Fix the code to check for the gcc version in two
more places, and change the existing checks to
use a convenience macro.

apt-pkg/cacheset.h

@@ -644,7 +644,7 @@ template<> inline void PackageContainer<std::vector<pkgCache::PkgIterator> >::in
 	for (const_iterator p = begin; p != end; ++p)
 		_cont.push_back(*p);
 }
-#if __cplusplus < 201103L
+#if APT_GCC_VERSION < 0x409
 template<> inline PackageContainer<std::set<pkgCache::PkgIterator> >::iterator PackageContainer<std::set<pkgCache::PkgIterator> >::erase(iterator i) {
 	_cont.erase(i._iter);
 	return end();
@@ -897,7 +897,7 @@ public:									/*{{{*/
 	bool empty() const APT_OVERRIDE { return _cont.empty(); }
 	void clear() APT_OVERRIDE { return _cont.clear(); }
 	size_t size() const APT_OVERRIDE { return _cont.size(); }
-#if __GNUC__ >= 5 || (__GNUC_MINOR__ >= 9 && __GNUC__ >= 4)
+#if APT_GCC_VERSION >= 0x409
 	iterator erase( const_iterator pos ) { return iterator(_cont.erase(pos._iter)); }
 	iterator erase( const_iterator first, const_iterator last ) { return iterator(_cont.erase(first._iter, last._iter)); }
 #else
@@ -1148,7 +1148,7 @@ template<> inline void VersionContainer<std::vector<pkgCache::VerIterator> >::in
 	for (const_iterator v = begin; v != end; ++v)
 		_cont.push_back(*v);
 }
-#if __cplusplus < 201103L
+#if APT_GCC_VERSION < 0x409
 template<> inline VersionContainer<std::set<pkgCache::VerIterator> >::iterator VersionContainer<std::set<pkgCache::VerIterator> >::erase(iterator i) {
 	_cont.erase(i._iter);
 	return end();

test/integration/header-compilation

@@ -0,0 +1,26 @@
+#!/bin/sh
+set -e
+
+TESTDIR=$(readlink -f $(dirname $0))
+. $TESTDIR/framework
+setupenvironment
+
+(for i in $TESTDIR/../../build/include/apt-pkg/*.h; do echo "#include \"$i\""; done) > include.h
+
+
+testone() {
+    local cxx=$1
+    shift
+
+    if type $cxx 2>&1 1>/dev/null; then
+        testsuccess $cxx -c -o /dev/null include.h
+        testsuccess $cxx -c -o /dev/null -std=c++98 include.h
+        testsuccess $cxx -c -o /dev/null -std=c++11 include.h
+    else
+        msgskip "Compile $cxx is not installed"
+    fi
+}
+
+testone clang++
+testone g++
+testone g++-4.8