Move build to Ubuntu 14.04 and GCC 7.2.

To be able to run on the same distributions as before we need to have
the same GLIBC version dependency as in Ubuntu 12.04, which is 2.15.

For that we need to remove all usages of GLIBC features from 2.16 and above.
Currently there are three methods used, so they're wrapped in a separate
static library, linux_glibc_wraps.

It is a separate library because it must be compiled without '-flto' flag,
otherwise the inline __asm__ is not working and we get unresolved symbols.

Telegram/SourceFiles/core/basic_types.h

@@ -26,25 +26,10 @@ Copyright (c) 2014-2017 John Preston, https://desktop.telegram.org
 #include <ctime>
 
 #include "base/build_config.h"
+#include "base/ordered_set.h"
 
 using gsl::not_null;
 
-#if defined COMPILER_GCC
-namespace std {
-
-template <typename T>
-constexpr std::add_const_t<T>& as_const(T& t) noexcept {
-    return t;
-}
-
-template <typename T>
-void as_const(const T&&) = delete;
-
-} // namespace std
-#endif // COMPILER_GCC
-
-#include "base/ordered_set.h"
-
 //using uchar = unsigned char; // Qt has uchar
 using int16 = qint16;
 using uint16 = quint16;