Fix test assumption that Linux implies glibc.

[danalbert]

Summary

This fixes an regression when using bionic introduced in r345173.

I need to follow up and figure out what exactly is implied by
TEST_HAS_C11_FEATURES and see what the correct configuration is for
bionic (new versions should have everything the tests care about,
versions that predate C11 certainly don't), but this gets the tests
back to the old behavior.

[mclow.lists]

I hate this chunk of code. :-(
TEST_HAS_C11_FEATURES basically means that we can use C11 library features like aligned_alloc, and timespec etc.

test/support/test_macros.h

 // This is deliberately different than _LIBCPP_STD_VER to discourage matching them up.
 #endif
 #endif
 
 // Attempt to deduce the GLIBC version
 #if (defined(__has_include) && __has_include(<features.h>)) || \
     defined(__linux__)
 #include <features.h>
+#if defined(__GLIBC_PREREQ)
 #define TEST_HAS_GLIBC
 #define TEST_GLIBC_PREREQ(major, minor) __GLIBC_PREREQ(major, minor)
 #endif
+#endif
 
 #if TEST_STD_VER >= 11
 #define TEST_ALIGNOF(...) alignof(__VA_ARGS__)
 #define TEST_ALIGNAS(...) alignas(__VA_ARGS__)
 #define TEST_CONSTEXPR constexpr
 #define TEST_NOEXCEPT noexcept
 #define TEST_NOEXCEPT_FALSE noexcept(false)
 #define TEST_NOEXCEPT_COND(...) noexcept(__VA_ARGS__)

 #  if defined(__FreeBSD__)
 //  Specifically, FreeBSD does NOT have timespec_get, even though they have all
 //  the rest of C11 - this is PR#38495
 #    define TEST_HAS_C11_FEATURES
 #  elif defined(__Fuchsia__)
 #    define TEST_HAS_C11_FEATURES
 #    define TEST_HAS_TIMESPEC_GET
 #  elif defined(__linux__)
-#    if !defined(_LIBCPP_HAS_MUSL_LIBC)
-#      if defined(TEST_GLIBC_PREREQ) && TEST_GLIBC_PREREQ(2, 17)
+// This block preserves the old behavior used by include/__config:
+// _LIBCPP_GLIBC_PREREQ would be defined to 0 if __GLIBC_PREREQ was not
+// available. The configuration here may be too vague though, as Bionic, uClibc,
+// newlib, etc may all support these features but need to be configured.
+#    if defined(TEST_GLIBC_PREREQ)
+#      if TEST_GLIBC_PREREQ(2, 17)
 #        define TEST_HAS_TIMESPEC_GET
 #        define TEST_HAS_C11_FEATURES
 #      endif
-#    else // defined(_LIBCPP_HAS_MUSL_LIBC)
+#    elif defined(_LIBCPP_HAS_MUSL_LIBC)
 #      define TEST_HAS_C11_FEATURES
 #      define TEST_HAS_TIMESPEC_GET
 #    endif
 #  elif defined(_WIN32)
 #    if defined(_MSC_VER) && !defined(__MINGW32__)
 #      define TEST_HAS_C11_FEATURES // Using Microsoft's C Runtime library
 #      define TEST_HAS_TIMESPEC_GET
 #    endif