prefer vertical lists rather than horizontal

Vertical lists, e.g., one string per line, can be version controlled
individually.  This commit also removes -Wpointer-arith duplicate from
my_CFLAGS variable.
diff --git a/configure.ac b/configure.ac
index 1926d18..e883619 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6,13 +6,27 @@
         [http://www.example.com/abc/])
 AC_CONFIG_SRCDIR([src/libabc.c])
 AC_CONFIG_AUX_DIR([build-aux])
-AM_INIT_AUTOMAKE([check-news foreign 1.11 -Wall -Wno-portability silent-rules tar-pax no-dist-gzip dist-xz subdir-objects])
+AM_INIT_AUTOMAKE([
+	check-news
+	foreign
+	1.11
+	-Wall
+	-Wno-portability
+	silent-rules
+	tar-pax
+	no-dist-gzip
+	dist-xz
+	subdir-objects
+])
 AC_PROG_CC_STDC
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 AC_CONFIG_MACRO_DIR([m4])
 AM_SILENT_RULES([yes])
-LT_INIT([disable-static pic-only])
+LT_INIT([
+	disable-static
+	pic-only
+])
 AC_PREFIX_DEFAULT([/usr])
 
 AC_PROG_SED
@@ -32,14 +46,24 @@
         AC_DEFINE(ENABLE_DEBUG, [1], [Debug messages.])
 ])
 
-AC_CHECK_FUNCS([__secure_getenv secure_getenv])
+AC_CHECK_FUNCS([ \
+	__secure_getenv \
+	secure_getenv\
+])
 
-my_CFLAGS="-Wall \
--Wmissing-declarations -Wmissing-prototypes \
--Wnested-externs -Wpointer-arith \
--Wpointer-arith -Wsign-compare -Wchar-subscripts \
--Wstrict-prototypes -Wshadow \
--Wformat-security -Wtype-limits"
+my_CFLAGS="\
+-Wall \
+-Wchar-subscripts \
+-Wformat-security \
+-Wmissing-declarations \
+-Wmissing-prototypes \
+-Wnested-externs \
+-Wpointer-arith \
+-Wshadow \
+-Wsign-compare \
+-Wstrict-prototypes \
+-Wtype-limits \
+"
 AC_SUBST([my_CFLAGS])
 
 AC_CONFIG_HEADERS(config.h)