configure: Fix --disable-charset

Somehow I ended up only testing the automatic detection and the error message
from --enable-charset. I totally forgot that --disable-charset should skip the
whole check.

Thanks to Robby for reporting this.

Signed-off-by: Uli Schlachter <psychon@znc.in>

configure.ac

@@ -580,19 +580,22 @@ AC_ARG_ENABLE( [charset],
 	AS_HELP_STRING([--disable-charset], [disable ICU support]),
 	[HAVE_ICU="$enableval"],
 	[HAVE_ICU="auto"])
-old_HAVE_ICU="$HAVE_ICU"
-PKG_CHECK_MODULES([icu], [icu-uc], [
-	appendLib "$icu_LIBS"
-	appendCXX "$icu_CFLAGS"
-	HAVE_ICU=yes
-	AC_DEFINE([HAVE_ICU], [1], [Enable ICU library for Unicode handling])
-	AC_DEFINE([U_USING_ICU_NAMESPACE], [0], [Do not clutter global namespace with ICU C++ stuff])
-], [
-	ZNC_AUTO_FAIL([HAVE_ICU],
-		      [support for charset conversion not found. Try --disable-charset.],
-		      [support for charset conversion not found and thus disabled])
-	HAVE_ICU="no (icu-uc not found via pkg-config)"
-])
+if test "x$HAVE_ICU" != "xno"
+then
+	old_HAVE_ICU="$HAVE_ICU"
+	PKG_CHECK_MODULES([icu], [icu-uc], [
+		appendLib "$icu_LIBS"
+		appendCXX "$icu_CFLAGS"
+		HAVE_ICU=yes
+		AC_DEFINE([HAVE_ICU], [1], [Enable ICU library for Unicode handling])
+		AC_DEFINE([U_USING_ICU_NAMESPACE], [0], [Do not clutter global namespace with ICU C++ stuff])
+	], [
+		ZNC_AUTO_FAIL([HAVE_ICU],
+			      [support for charset conversion not found. Try --disable-charset.],
+			      [support for charset conversion not found and thus disabled])
+		HAVE_ICU="no (icu-uc not found via pkg-config)"
+	])
+fi
 
 AC_CACHE_CHECK([for GNU make], [ac_cv_path_GNUMAKE], [
 	AC_PATH_PROGS_FEATURE_CHECK([GNUMAKE], [make gmake], [[