wmbattery: Use AC_CHECK_LIB instead of AC_CHECK_HEADERS to check for libapm.

If autoconf finds an apm.h other than the one belonging to libapm, we get
builds errors like the ones reported by Haroldo Gambini Santos in [1].

[1] http://lists.windowmaker.org/dev/msg07937.html

wmbattery/apm.h

@@ -8,7 +8,7 @@
 #include <i386/apmvar.h>
 #endif
 
-#ifdef HAVE_APM_H
+#ifdef HAVE_LIBAPM
 #include <apm.h>
 #endif
 
@@ -37,7 +37,7 @@
 #define BATTERY_TIME_UNKNOWN        (-1)
 #endif /* AC_LINE_STATUS_ON */
 
-#if !defined(HAVE_APM_H)
+#if !defined(HAVE_LIBAPM)
 typedef struct {
 	const char driver_version[10];
 	int apm_version_major;

wmbattery/configure.ac

@@ -34,7 +34,6 @@ AC_HEADER_STDC
 AC_CHECK_HEADERS(X11/xpm.h)
 AC_CHECK_HEADERS(X11/extensions/shape.h)
 AC_CHECK_HEADERS(getopt.h)
-AC_CHECK_HEADERS(apm.h)
 dnl FreeBSD needs apm_bios.h
 AC_CHECK_HEADERS(machine/apm_bios.h)
 dnl NetBSD and OpenBSD need apmvar.h

wmbattery/wmbattery.c

@@ -148,7 +148,7 @@ int apm_exists(void)
 	return apm_read(&i);
 }
 #endif
-#if !defined(HAVE_APM_H)
+#if !defined(HAVE_LIBAPM)
 int apm_read(apm_info *i)
 {
 	return -1;