configure: rename from --enable-gtp-kernel to --enable-gtp-linux

This software seems to run on other *nixes, so indicate explicitly that
this is the Linux kernel datapath support.
diff --git a/configure.ac b/configure.ac
index 66c4892..9b8d988 100644
--- a/configure.ac
+++ b/configure.ac
@@ -45,16 +45,16 @@
 # FIXME: Replace `main' with a function in `-links':
 #AC_CHECK_LIB([inks], [main])
 
-dnl GTP kernel dependencies
-AC_ARG_ENABLE([gtp-kernel],
-	AS_HELP_STRING([--enable-gtp-kernel], [Build GTP tunneling kernel]),
-	[enable_gtp_kernel="$enableval"], [enable_gtp_kernel="no"])
+dnl GTP Linux kernel dependencies
+AC_ARG_ENABLE([gtp-linux],
+	AS_HELP_STRING([--enable-gtp-linux], [Build GTP tunneling Linux kernel]),
+	[enable_gtp_linux="$enableval"], [enable_gtp_linux="no"])
 
-AS_IF([test "x$enable_gtp_kernel" = "xyes"], [
+AS_IF([test "x$enable_gtp_linux" = "xyes"], [
 	PKG_CHECK_MODULES([LIBGTPNL], [libgtpnl >= 1.0.0])
 ])
 
-AM_CONDITIONAL([ENABLE_GTP_KERNEL], [test "$enable_gtp_kernel" = "yes"])
+AM_CONDITIONAL([ENABLE_GTP_KERNEL], [test "$enable_gtp_linux" = "yes"])
 
 # Checks for header files.
 AC_HEADER_STDC
@@ -133,4 +133,4 @@
 
 echo "
 openggsn Configuration:
-  GTP kernel support:			${enable_gtp_kernel}"
+  GTP Linux kernel support:			${enable_gtp_linux}"