Fix PKG_CHECK_MODULES name

BUG=none
TEST=builds clean with message:
  checking for X11... yes

Change-Id: I7d174680be0283df6d3c40449871342e2a6e0dfe
diff --git a/Makefile.am b/Makefile.am
index 84b40fb..9f9712c 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -3,6 +3,6 @@
 
 bin_PROGRAMS = mtplot
 
-mtplot_LDADD = $(MTPLOT_LIBS)
+mtplot_LDADD = $(X11_LIBS)
 
 EXTRA_DIST = INSTALL
diff --git a/configure.ac b/configure.ac
index 3c1ff7a..81b5ece 100644
--- a/configure.ac
+++ b/configure.ac
@@ -9,6 +9,6 @@
 AC_PROG_CC
 AC_PROG_INSTALL
 
-PKG_CHECK_MODULES(MTPLOT, [x11])
+PKG_CHECK_MODULES(X11, [x11])
 
 AC_OUTPUT([Makefile])