lib/icu/Compare: add fallback using strcasecmp() and strcasestr()

Our IcuCaseFold() fallback using strxfrm() is not actually case
insensitive.  This commit fixes the problem by switching to
strcasecmp().  That function is not guaranteed to support UTF-8, but
it's the best we can do in this sparse situation.

Closes #111

src/lib/icu/Compare.cxx

@@ -24,18 +24,43 @@
 
 #include <string.h>
 
+#ifdef HAVE_ICU_CASE_FOLD
+
 IcuCompare::IcuCompare(const char *_needle) noexcept
 	:needle(IcuCaseFold(_needle)) {}
 
+#else
+
+IcuCompare::IcuCompare(const char *_needle) noexcept
+	:needle(AllocatedString<>::Duplicate(_needle)) {}
+
+#endif
+
 bool
 IcuCompare::operator==(const char *haystack) const noexcept
 {
+#ifdef HAVE_ICU_CASE_FOLD
 	return StringIsEqual(IcuCaseFold(haystack).c_str(), needle.c_str());
+#else
+	return strcasecmp(haystack, needle.c_str());
+#endif
 }
 
 bool
 IcuCompare::IsIn(const char *haystack) const noexcept
 {
+#ifdef HAVE_ICU_CASE_FOLD
 	return StringFind(IcuCaseFold(haystack).c_str(),
 			  needle.c_str()) != nullptr;
+#elif defined(HAVE_STRCASESTR)
+	return strcasestr(haystack, needle.c_str()) != nullptr;
+#else
+	/* poor man's strcasestr() */
+	for (const size_t length = strlen(needle.c_str());
+	     *haystack != 0; ++haystack)
+		if (strncasecmp(haystack, needle.c_str(), length) == 0)
+			return true;
+
+	return false;
+#endif
 }