Do not touch CFLAGS in configure.ac

This fixes an inconsistency between tests run by ./configure and actual make
diff --git a/configure.ac b/configure.ac
index c922b30..a1d8152 100644
--- a/configure.ac
+++ b/configure.ac
@@ -71,6 +71,7 @@
 AM_PROG_CC_C_O
 m4_version_prereq([2.70], [], [AC_PROG_CC_C99])
 AS_IF([test "x$ac_cv_prog_cc_c99" = xno], [AC_MSG_ERROR([htop is written in C99. A newer compiler is required.])])
+AM_CFLAGS="-std=c99 -pedantic"
 
 # ----------------------------------------------------------------------
 
@@ -349,15 +350,15 @@
       htop_config_script_cflags=$([$4] --cflags 2> /dev/null)
    fi
    htop_script_success=no
-   htop_save_CFLAGS="$CFLAGS"
+   htop_save_CFLAGS="$AM_CFLAGS"
    if test ! "x$htop_config_script_libs" = x; then
-      CFLAGS="$htop_config_script_cflags $CFLAGS"
+      AM_CFLAGS="$AM_CFLAGS $htop_config_script_cflags"
       AC_CHECK_LIB([$1], [$2], [
          AC_DEFINE([$3], 1, [The library is present.])
          LIBS="$htop_config_script_libs $LIBS "
          htop_script_success=yes
       ], [
-         CFLAGS="$htop_save_CFLAGS"
+         AM_CFLAGS="$htop_save_CFLAGS"
       ], [
          $htop_config_script_libs
       ])
@@ -503,7 +504,7 @@
    yes)
       AC_CHECK_LIB([unwind], [backtrace], [], [AC_MSG_ERROR([can not find required library libunwind])])
       AC_CHECK_HEADERS([libunwind.h], [], [
-         CFLAGS="$CFLAGS -I/usr/include/libunwind"
+         AM_CFLAGS="$AM_CFLAGS -I/usr/include/libunwind"
          AC_CHECK_HEADERS([libunwind/libunwind.h], [], [AC_MSG_ERROR([can not find required header file libunwind.h])])
       ])
       ;;
@@ -529,7 +530,7 @@
       m4_ifdef([PKG_PROG_PKG_CONFIG], [
          PKG_PROG_PKG_CONFIG()
          PKG_CHECK_MODULES(HWLOC, hwloc, [
-               CFLAGS="$CFLAGS $HWLOC_CFLAGS"
+               AM_CFLAGS="$AM_CFLAGS $HWLOC_CFLAGS"
                LIBS="$LIBS $HWLOC_LIBS"
                AC_DEFINE([HAVE_LIBHWLOC], [1], [Define to 1 if you have the 'hwloc' library (-lhwloc).])
             ], [
@@ -654,7 +655,7 @@
             PKG_CHECK_MODULES(LIBNL3, libnl-3.0, [], [enable_delayacct=no])
             PKG_CHECK_MODULES(LIBNL3GENL, libnl-genl-3.0, [], [enable_delayacct=no])
             if test "$enable_delayacct" = yes; then
-               CFLAGS="$CFLAGS $LIBNL3_CFLAGS $LIBNL3GENL_CFLAGS"
+               AM_CFLAGS="$AM_CFLAGS $LIBNL3_CFLAGS $LIBNL3GENL_CFLAGS"
                LIBS="$LIBS $LIBNL3_LIBS $LIBNL3GENL_LIBS"
                AC_DEFINE([HAVE_DELAYACCT], [1], [Define if delay accounting support should be enabled.])
             fi
@@ -669,7 +670,7 @@
          PKG_PROG_PKG_CONFIG()
          PKG_CHECK_MODULES(LIBNL3, libnl-3.0, [], [AC_MSG_ERROR([can not find required library libnl3])])
          PKG_CHECK_MODULES(LIBNL3GENL, libnl-genl-3.0, [], [AC_MSG_ERROR([can not find required library libnl3genl])])
-         CFLAGS="$CFLAGS $LIBNL3_CFLAGS $LIBNL3GENL_CFLAGS"
+         AM_CFLAGS="$AM_CFLAGS $LIBNL3_CFLAGS $LIBNL3GENL_CFLAGS"
          LIBS="$LIBS $LIBNL3_LIBS $LIBNL3GENL_LIBS"
          AC_DEFINE([HAVE_DELAYACCT], [1], [Define if delay accounting support should be enabled.])
       ], [
@@ -720,7 +721,7 @@
 # Checks for compiler warnings.
 # ----------------------------------------------------------------------
 
-AM_CFLAGS="\
+AM_CFLAGS="$AM_CFLAGS\
  -Wall\
  -Wcast-align\
  -Wcast-qual\