Make the compiler requirement clearer.

* configure.ac: Check for g++ 4.6 bugs, so we catch
the error at compile time, not make time.
* README: Mention the minimal g++ and clang++ versions.

README

@@ -29,6 +29,7 @@ Requirements
 ------------
 
 Spot requires a C++11-compliant compiler.
+G++ 4.8 or later, as well as Clang++3.5 or later should work.
 
 Spot expects a complete installation of Python (version 2.0 or later).
 Especially, Python's headers files should be installed.  If you don't
@@ -38,8 +39,6 @@ configure with the --disable-python option (see below).
 Optional third-party dependencies
 ----------------------------------
 
-
-
 If the SAT-solver glucose is found on your system, it will
 be used by our test suite to test our SAT-based minimization
 algorithm.

configure.ac

@@ -75,6 +75,8 @@ CXXFLAGS="$CXXFLAGS -DSPOT_BUILD"
 m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody],
 [AC_LANG_SOURCE([#include <memory>
 #include <string>
+#include <chrono>  // fails with some installation of clang
+#include <map>
 
    template <typename T>
     struct check
@@ -94,6 +96,12 @@ m4_define([_AX_CXX_COMPILE_STDCXX_11_testbody],
     check_type&& cr = static_cast<check_type&&>(c);
 
     auto d = a;
+
+    void test_emplace()
+    {
+       std::map<int, int> m;
+       m.emplace(1, 2); // fails with g++ 4.6
+    }
 ])])
 
 for f in -std=c++11 '-std=c++11 -stdlib=libc++' -std=c++0x