[refactor] reorganize configure, use sys extension

Our code relies on _GNU_SOURCE in a couple of places, so make sure we
call it. Take this opportunity to reorder and clean up the configure.ac
config a bit.

configure.ac

-# see semver.org for version info
+# See semver.org for version info
+#################################
+
 m4_define([VERSION_MAJOR], [0])
 m4_define([VERSION_MINOR], [1])
 m4_define([VERSION_PATCH], [0])
 m4_define([VERSION_STRING], VERSION_MAJOR.VERSION_MINOR.VERSION_PATCH)
 
-AC_INIT([aml],[VERSION_STRING],[swann@anl.gov])
-
-AC_SUBST([PACKAGE_VERSION_MAJOR],[VERSION_MAJOR])
-AC_SUBST([PACKAGE_VERSION_MINOR],[VERSION_MINOR])
-AC_SUBST([PACKAGE_VERSION_PATCH],[VERSION_PATCH])
+# Init build tools
+##################
 
-# are we in the right source dir ?
+AC_INIT([aml],[VERSION_STRING],[swann@anl.gov])
 AC_CONFIG_SRCDIR([include/aml.h])
-
-# build artefacts in separate dir
 AC_CONFIG_AUX_DIR([m4])
 AC_CONFIG_MACRO_DIR([m4])
-
 # automake should fail on any error
 AM_INIT_AUTOMAKE([-Wall -Werror foreign subdir-objects 1.12])
-AM_PROG_AR
 
-# check for libtool
-LT_INIT
+# Detect features
+#################
 
-# check for programs
 AC_LANG([C])
+AC_USE_SYSTEM_EXTENSIONS
 AC_PROG_CC
-AC_PROG_CC_STDC
+AC_PROG_CC_C99
 AM_PROG_CC_C_O
 AC_PROG_CPP
 AC_TYPE_SIZE_T
 AC_TYPE_INTPTR_T
 
+AM_PROG_AR
+LT_INIT
+
+# Extra dependencies, configuration
+###################################
+
+AC_SUBST([PACKAGE_VERSION_MAJOR],[VERSION_MAJOR])
+AC_SUBST([PACKAGE_VERSION_MINOR],[VERSION_MINOR])
+AC_SUBST([PACKAGE_VERSION_PATCH],[VERSION_PATCH])
+
+
 # support for testing with valgrind
 AC_ARG_ENABLE(valgrind,
 [AS_HELP_STRING([--enable-valgrind],[Also valgrind on checks (default is no).])],
@@ -87,6 +93,8 @@ else
 fi
 AM_CONDITIONAL([BUILD_DOCS],[ test "x$BUILD_DOCS" = xyes ])
 
+# Output
+########
 
 AC_CONFIG_HEADERS([include/config.h])