build: MBIM support is auto

Use it if found, don't use it if not found.

Can be explicitly requested with --with-mbim.

configure.ac

@@ -171,21 +171,32 @@ case $with_tests in
 esac
 
 dnl
-dnl MBIM support (enabled by default)
+dnl MBIM support (auto by default)
 dnl
-AC_ARG_WITH(mbim, AS_HELP_STRING([--without-mbim], [Build without MBIM support]), [], [with_mbim=yes])
-AM_CONDITIONAL(WITH_MBIM, test "x$with_mbim" = "xyes")
-case $with_mbim in
-    yes)
-        PKG_CHECK_MODULES(MBIM, mbim-glib)
-        AC_DEFINE(WITH_MBIM, 1, [Define if you want MBIM support])
-        AC_SUBST(MBIM_CFLAGS)
-        AC_SUBST(MBIM_LIBS)
-        ;;
-    *)
-        with_mbim=no
-        ;;
-esac
+AC_ARG_WITH(mbim, AS_HELP_STRING([--without-mbim], [Build without MBIM support]), [], [with_mbim=auto])
+if (test "${with_mbim}" != "no"); then
+    PKG_CHECK_MODULES(MBIM,
+                      [mbim-glib],
+                      [have_libmbim_glib=yes],
+                      [have_libmbim_glib=no])
+    AC_SUBST(MBIM_CFLAGS)
+    AC_SUBST(MBIM_LIBS)
+
+    if (test "${have_libmbim_glib}" = "no"); then
+        if (test "${with_mbim}" = "yes"); then
+            AC_MSG_ERROR([Couldn't find libmbim-glib])
+        else
+            with_mbim="no"
+        fi
+    else
+        with_mbim="yes"
+    fi
+fi
+
+if (test "${with_mbim}" = "yes"); then
+    AC_DEFINE(WITH_MBIM, 1, [Define if you want MBIM support])
+fi
+AM_CONDITIONAL(WITH_MBIM, test "${with_mbim}" = "yes")
 
 dnl
 dnl QMI support (enabled by default)