[libc++] Use runtime rather then compile-time glibc version check

[phosek]

Summary

glibc supports versioning, so it's possible to build against older
version and run against newer version. This is sometimes relied on
in practice, e.g. in Fuchsia build we build against older sysroot
(equivalent to Ubuntu Trusty) to cover the broadest possible range
of host systems, but that doesn't necessarily match the system that
binary is going to run on which may have newer version, in which case
the compile test used in curr_symbol is going to fail. Using runtime
check is more reliable.

[ldionne]

libcxx/test/std/localization/locale.categories/category.monetary/locale.moneypunct.byname/curr_symbol.pass.cpp

119–123

This logic could be defined in a function like:

#if defined(_CS_GNU_LIBC_VERSION)
bool glibc_version_less_than(char const* version) {
    std::string test_version = "glibc "; test_version += version;

    size_t n = confstr(_CS_GNU_LIBC_VERSION, nullptr, (size_t)0);
    char *current_version = new char[n];
    confstr(_CS_GNU_LIBC_VERSION, current_version, n);
    
    bool result = strverscmp(current_version, test_version.c_str()) < 0;
    delete[] current_version;
    return result;
}
#endif

Then, we could use it as:

#if defined(_CS_GNU_LIBC_VERSION)
    // GLIBC <= 2.23 uses currency_symbol="<U0440><U0443><U0431>"
    // GLIBC >= 2.24 uses currency_symbol="<U20BD>"
    // See also: http://www.fileformat.info/info/unicode/char/20bd/index.htm
    if (!glibc_version_less_than("2.24")) {
        assert(f.curr_symbol() == " \u20BD");
    } else {
        assert(f.curr_symbol() == " \xD1\x80\xD1\x83\xD0\xB1");
    }
#else
    assert(f.curr_symbol() == " \xD1\x80\xD1\x83\xD0\xB1");
#endif

We may or may not want to lift this function to a header common to the tests, but not defining the logic inline seems like a gain in readability. WDYT?

[phosek]

Ping?

libcxx/test/std/localization/locale.categories/category.monetary/locale.moneypunct.byname/curr_symbol.pass.cpp

int main()
         Fwf f(LOCALE_fr_FR_UTF_8, 1);
         assert(f.curr_symbol() == L" \u20ac");
     }
     {
         Fwt f(LOCALE_fr_FR_UTF_8, 1);
         assert(f.curr_symbol() == L" EUR");
     }
 
+#if defined(_CS_GNU_LIBC_VERSION)
+    size_t n = confstr(_CS_GNU_LIBC_VERSION, nullptr, (size_t)0);
+    char *buf = new char[n];
+    confstr(_CS_GNU_LIBC_VERSION, buf, n);
+    bool glibc_2_24_currency_symbol = strverscmp(buf, "glibc 2.24") >= 0;
+    delete[] buf;
+#endif
+
     {
         Fnf f(LOCALE_ru_RU_UTF_8, 1);
+#if defined(_CS_GNU_LIBC_VERSION)
         // GLIBC <= 2.23 uses currency_symbol="<U0440><U0443><U0431>"
         // GLIBC >= 2.24 uses currency_symbol="<U20BD>"
         // See also: http://www.fileformat.info/info/unicode/char/20bd/index.htm
-#if defined(TEST_GLIBC_PREREQ)
-    #if TEST_GLIBC_PREREQ(2, 24)
-        #define TEST_GLIBC_2_24_CURRENCY_SYMBOL
-    #endif
-#endif
-
-#if defined(TEST_GLIBC_2_24_CURRENCY_SYMBOL)
+        if (glibc_2_24_currency_symbol)
           assert(f.curr_symbol() == " \u20BD");
+        else
+          assert(f.curr_symbol() == " \xD1\x80\xD1\x83\xD0\xB1");
 #else
         assert(f.curr_symbol() == " \xD1\x80\xD1\x83\xD0\xB1");
 #endif
     }
     {
         Fnt f(LOCALE_ru_RU_UTF_8, 1);
         assert(f.curr_symbol() == " RUB");
     }
     {
         Fwf f(LOCALE_ru_RU_UTF_8, 1);
-#if defined(TEST_GLIBC_2_24_CURRENCY_SYMBOL)
+#if defined(_CS_GNU_LIBC_VERSION)
+        if (glibc_2_24_currency_symbol)
           assert(f.curr_symbol() == L" \u20BD");
+        else
+          assert(f.curr_symbol() == L" \x440\x443\x431");
 #else
         assert(f.curr_symbol() == L" \x440\x443\x431");
 #endif
     }
 
     {
         Fwt f(LOCALE_ru_RU_UTF_8, 1);
         assert(f.curr_symbol() == L" RUB");