configure.ac: Use autoconf macro for GNU make.
diff --git a/configure.ac b/configure.ac
index 1d4a6ae..f36098d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -53,7 +53,7 @@
 AC_PROG_CXX
 AM_PROG_CC_C_O
 AM_PROG_AS
-AC_CHECK_PROGS([MAKE], [gmake make])
+AX_CHECK_GNU_MAKE
 AC_CHECK_PROGS([PYTHON2], [python2 python])
 AC_PROG_SED
 AC_PROG_MKDIR_P