build: require glibc 2.18+ for release builds

From what I can see the only platform this drops support for is CentOS
7. CentOS 7 reached the end of it's "full update" support at the end of
2020. It does receive maintenance updates until 2024, however I don't
think supporting glibc 2.17 until 2024 is realistic. Note that anyone
wanting to self-compile and target a glibc 2.17 runtime could build with
--disable-threadlocal.

glibc 2.18 was released in August 2013.
https://sourceware.org/legacy-ml/libc-alpha/2013-08/msg00160.html

contrib/devtools/test-symbol-check.py

@@ -44,7 +44,7 @@ class TestSymbolChecks(unittest.TestCase):
             self.skipTest("test not available for RISC-V")
 
         # nextup was introduced in GLIBC 2.24, so is newer than our supported
-        # glibc (2.17), and available in our release build environment (2.24).
+        # glibc (2.18), and available in our release build environment (2.24).
         with open(source, 'w', encoding="utf8") as f:
             f.write('''
                 #define _GNU_SOURCE