diff --git a/configure.ac b/configure.ac
index 0c7732c4..52227db0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -74,21 +74,21 @@ AC_CHECK_HEADERS([sys/sendfile.h])
 # optional components
 #
 AC_ARG_ENABLE([server],
-  AS_HELP_STRING([--disable-server], [disable building tntnet server]),
+  [AS_HELP_STRING([--disable-server], [disable building tntnet server])],
   [enable_server=$enableval],
   [enable_server=yes])
 
 AM_CONDITIONAL(MAKE_SERVER, test x$enable_server = xyes)
 
 AC_ARG_ENABLE([sdk],
-  AS_HELP_STRING([--disable-sdk], [disable building sdk]),
+  [AS_HELP_STRING([--disable-sdk], [disable building sdk])],
   [enable_sdk=$enableval],
   [enable_sdk=yes])
 
 AM_CONDITIONAL(MAKE_SDK, test x$enable_sdk = xyes)
 
 AC_ARG_ENABLE([demos],
-  AS_HELP_STRING([--enable-demos], [enable building demos]),
+  [AS_HELP_STRING([--enable-demos], [enable building demos])],
   [enable_demos=$enableval],
   [enable_demos=no])
 
@@ -105,17 +105,16 @@ AS_IF([test "$enable_demos" = "yes"],
 )
 
 AC_ARG_WITH([md2man],
-  AS_HELP_STRING([--with-md2man], [use go-md2man tool to generate man pages from markdown])
+  [AS_HELP_STRING([--with-md2man], [use go-md2man tool to generate man pages from markdown])],
   [with_md2man=$withval])
 
 AS_IF(
   [test "$with_md2man"],
-  AC_CHECK_PROG(MD2MAN, [go-md2man], [go-md2man])
+  [AC_CHECK_PROG(MD2MAN, [go-md2man], [go-md2man])
   AS_IF(
     [test -z "$MD2MAN"],
     AC_MSG_ERROR([go-md2man not found])
-  )
-
+  )]
 )
 
 AM_CONDITIONAL(GENERATE_MAN, test ! -z "$with_md2man")
@@ -125,21 +124,15 @@ AM_CONDITIONAL(GENERATE_MAN, test ! -z "$with_md2man")
 #
 AC_CHECK_FUNCS([setenv])
 
-case "${host_cpu}-${host_os}" in
-*-aix*)
-    SHARED_LIB_FLAG=-qmkshrobj
-    ;;
-*-darwin*)
-    SHARED_LIB_FLAG=-dynamiclib
-    ;;
-*)
-    SHARED_LIB_FLAG=-shared
-    ;;
-esac
+AS_CASE(["${host_cpu}-${host_os}"],
+    [*-aix*], [SHARED_LIB_FLAG=-qmkshrobj],
+    [*-darwin*], [SHARED_LIB_FLAG=-dynamiclib],
+    [SHARED_LIB_FLAG=-shared]
+)
 AC_SUBST(SHARED_LIB_FLAG)
 
 AC_ARG_ENABLE([unittest],
-  AS_HELP_STRING([--disable-unittest], [disable unittest]),
+  [AS_HELP_STRING([--disable-unittest], [disable unittest])],
   [enable_unittest=$enableval],
   [enable_unittest=enable_unittest])