Avoid referring to __strspn_c1 when compiling on older glibc

* See #2406

Author: eregon

CHANGELOG.md

@@ -15,6 +15,7 @@ Bug fixes:
 * Fix `rb_str_modify_expand` to preserve existing bytes (#2392).
 * Fix `Marshal.load` of multiple `Symbols` with an explicit encoding (#1624).
 * Fix `String#scrub` when replacement is frozen (#2398, @LillianZ).
+* Fix issue with `strspn` used in the `date` C extension compiled as a macro on older glibc and then missing the `__strspn_c1` symbol on newer glibc (#2406).
 
 Compatibility:
 

src/main/c/date/date_strftime.c

@@ -16,6 +16,11 @@
 #include <sys/time.h>
 #endif
 
+// TruffleRuby: strspn is a macro on old glibc, and was removed in https://sourceware.org/legacy-ml/libc-alpha/2016-03/msg00772.html
+// undef the macro so the function is always used instead, and builds on older glibc work with newer glibc.
+// See https://github.com/oracle/truffleruby/issues/2406
+#undef strspn
+
 #undef strchr	/* avoid AIX weirdness */
 
 #define range(low, item, hi)	(item)