Test for Linux-specific sysinfo() only on Linux systems.

Some other systems have sysinfo() with different semantics.

m4/lc_physmem.m4

@@ -54,6 +54,10 @@ main(void)
 		with sysctl().])
 	AC_MSG_RESULT([sysctl])
 ], [
+dnl sysinfo() is Linux-specific. Some non-Linux systems have
+dnl incompatible sysinfo() so we must check $host_os.
+case $host_os in
+	linux*)
 		AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
 #include <sys/sysinfo.h>
 int
@@ -65,10 +69,16 @@ main(void)
 }
 		]])], [
 			AC_DEFINE([HAVE_PHYSMEM_SYSINFO], [1],
-		[Define to 1 if the amount of physical memory can be detected
-		with sysinfo().])
+				[Define to 1 if the amount of physical memory
+				can be detected with Linux sysinfo().])
 			AC_MSG_RESULT([sysinfo])
 		], [
 			AC_MSG_RESULT([unknown])
-])])])
+		])
+		;;
+	*)
+		AC_MSG_RESULT([unknown])
+		;;
+esac
+])])
 ])dnl lc_PHYSMEM