configure.ac: fix: properly check whether to enable ms-trx

Condition 'test ENABLE_MS_TRX' is always true because 'ENABLE_MS_TRX'
is a string literal, not a variable...  Because of this, automeke will
unconditionally try to configure the submodule.

Change-Id: Icbb9278c688bfe506d5ad726f16a6c200572de1b
Fixes: b7253c6fdc88a3e2e0c8b441dc3c34dfb1c15d24
Related: OS#5599
diff --git a/configure.ac b/configure.ac
index 699fb32..ff1ae8b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -334,7 +334,7 @@
 AC_MSG_RESULT([CXXFLAGS="$CXXFLAGS"])
 AC_MSG_RESULT([LDFLAGS="$LDFLAGS"])
 
-if test ENABLE_MS_TRX; then
+if test "x$with_mstrx" = "xyes"; then
     AC_MSG_NOTICE(["Enabling ms-trx..."])
     AC_CONFIG_SUBDIRS([osmocom-bb/src/host/trxcon])
 fi