Reorder configure macros to avoid "missing script" warning.

3 effects in this commit, with the first being the main one:

1. Fix the "`missing' script is too old or missing" warning. See:
   <https://lists.gnu.org/archive/html/automake/2010-08/msg00108.html>

2. By moving AC_CANONICAL_TARGET down in order, we are now able to
   set the directory for auxiliary scripts. For now it's still './'.
   I added the line "AC_CONFIG_AUX_DIR([.])" to show that the directory
   change is possible.

3. AC_USE_SYSTEM_EXTENSIONS includes checks from AC_PROG_CC, by moving
   the former macro down, we can save size in 'configure' by not
   generating repeated checks.

configure.ac

@@ -7,20 +7,24 @@ AC_INIT([htop],[2.0.1],[hisham@gobolinux.org])
 SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(date +%s)}"
 year=$(date -u -d "@$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u -r "$SOURCE_DATE_EPOCH" "+%Y" 2>/dev/null || date -u "+%Y")
 
-# The following two lines are required by hwloc scripts
-AC_USE_SYSTEM_EXTENSIONS
-AC_CANONICAL_TARGET
-
-AM_INIT_AUTOMAKE([1.11])
 AC_CONFIG_SRCDIR([htop.c])
+AC_CONFIG_AUX_DIR([.])
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4])
 
+# Required by hwloc scripts
+AC_CANONICAL_TARGET
+
+AM_INIT_AUTOMAKE([1.11])
+
 # Checks for programs.
 # ----------------------------------------------------------------------
 AC_PROG_CC
 AM_PROG_CC_C_O
 
+# Required by hwloc scripts
+AC_USE_SYSTEM_EXTENSIONS
+
 LT_INIT([disable-shared static])
 
 # Checks for platform.