Add support for ICU 4.2

Supporting ICU 4.2 seems useful because it ships with CentOS 6.

Versions before ICU 4.6 don't support pkg-config, so document an
installation method without using pkg-config.

In ICU 4.2, ucol_getKeywordsForLocale() sometimes returns values that
will not be accepted by uloc_toLanguageTag().  Skip loading keyword
variants in that version.
Reported-by: Victor Wagner <vitus@wagner.pp.ru>

doc/src/sgml/installation.sgml

@@ -774,10 +774,26 @@ su - postgres
          Build with support for
          the <productname>ICU</productname><indexterm><primary>ICU</></>
          library.  This requires the <productname>ICU4C</productname> package
-         as well
-         as <productname>pkg-config</productname><indexterm><primary>pkg-config</></>
          to be installed.  The minimum required version
-         of <productname>ICU4C</productname> is currently 4.6.
+         of <productname>ICU4C</productname> is currently 4.2.
+        </para>
+
+        <para>
+         By default,
+         <productname>pkg-config</productname><indexterm><primary>pkg-config</></>
+         will be used to find the required compilation options.  This is
+         supported for <productname>ICU4C</productname> version 4.6 and later.
+         For older versions, or if <productname>pkg-config</productname> is
+         not available, the variables <envar>ICU_CFLAGS</envar>
+         and <envar>ICU_LIBS</envar> can be specified
+         to <filename>configure</filename>, like in this example:
+<programlisting>
+./configure ... --with-icu ICU_CFLAGS='-I/some/where/include' ICU_LIBS='-L/some/where/lib -licui18n -licuuc -licudata'
+</programlisting>
+         (If <productname>ICU4C</productname> is in the default search path
+         for the compiler, then you still need to specify a nonempty string in
+         order to avoid use of <productname>pkg-config</productname>, for
+         example, <literal>ICU_CFLAGS=' '</literal>.)
         </para>
        </listitem>
       </varlistentry>

src/backend/commands/collationcmds.c

@@ -722,7 +722,17 @@ pg_import_system_collations(PG_FUNCTION_ARGS)
 
 			/*
 			 * Add keyword variants
+			 *
+			 * In ICU 4.2, ucol_getKeywordsForLocale() sometimes returns
+			 * values that will not be accepted by uloc_toLanguageTag().  Skip
+			 * loading keyword variants in that version.  (Both
+			 * ucol_getKeywordValuesForLocale() and uloc_toLanguageTag() are
+			 * new in ICU 4.2, so older versions are not supported at all.)
+			 *
+			 * XXX We have no information about ICU 4.3 through 4.7, but we
+			 * know the below works with 4.8.
 			 */
+#if U_ICU_VERSION_MAJOR_NUM > 4 || (U_ICU_VERSION_MAJOR_NUM == 4 && U_ICU_VERSION_MINOR_NUM > 2)
 			status = U_ZERO_ERROR;
 			en = ucol_getKeywordValuesForLocale("collation", name, TRUE, &status);
 			if (U_FAILURE(status))
@@ -769,6 +779,7 @@ pg_import_system_collations(PG_FUNCTION_ARGS)
 						(errmsg("could not get keyword values for locale \"%s\": %s",
 								name, u_errorName(status))));
 			uenum_close(en);
+#endif							/* ICU >4.2 */
 		}
 	}
 #endif							/* USE_ICU */