silence compiler warnings for gcc 4.7 and before

[DerDakon]

The option -Wliteral-suffix was introduced in gcc 4.8, and #pragma GCC diagnostic push was added in 4.6. Using the header with an older gcc would lead to warnings like:

string_view.hpp:438: warning: expected [error|warning|ignored] after '#pragma GCC diagnostic'
string_view.hpp:439: warning: unknown option after '#pragma GCC diagnostic' kind
string_view.hpp:1681: warning: expected [error|warning|ignored] after '#pragma GCC diagnostic'