Merge #43: build: minor build changes

7d3fe04 test: replace ` with ' (fanquake)
b77ca22 build: use AX_CHECK_COMPILE_FLAG to test for -fvisibility=hidden (fanquake)
6bbafb9 build: AX_CXX_COMPILE_STDCXX serial 12 (fanquake)
0ca1fef build: AX_CHECK_PREPROC_FLAG serial 6 (fanquake)
281411b build: AX_CHECK_COMPILE_FLAG serial 6 (fanquake)

Pull request description:

  This is very minor build changes, mostly updating external macros.

ACKs for top commit:
  sipa:
    utACK 7d3fe04

Tree-SHA512: dbd4cda1dd4a4f4ba8492ec89c868cb9187ac08707be1ada143e59c42a8023d6625ebea1ac51044c6f070d0f13a3ee174569238aaf11d6da1aa9367346809a1a

Author: sipa

build-aux/m4/ax_check_compile_flag.m4

@@ -1,5 +1,5 @@
 # ===========================================================================
-#   http://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -29,33 +29,12 @@
 #   Copyright (c) 2008 Guido U. Draheim <[email protected]>
 #   Copyright (c) 2011 Maarten Bosmans <[email protected]>
 #
-#   This program is free software: you can redistribute it and/or modify it
-#   under the terms of the GNU General Public License as published by the
-#   Free Software Foundation, either version 3 of the License, or (at your
-#   option) any later version.
-#
-#   This program is distributed in the hope that it will be useful, but
-#   WITHOUT ANY WARRANTY; without even the implied warranty of
-#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
-#   Public License for more details.
-#
-#   You should have received a copy of the GNU General Public License along
-#   with this program. If not, see <http://www.gnu.org/licenses/>.
-#
-#   As a special exception, the respective Autoconf Macro's copyright owner
-#   gives unlimited permission to copy, distribute and modify the configure
-#   scripts that are the output of Autoconf when processing the Macro. You
-#   need not follow the terms of the GNU General Public License when using
-#   or distributing such scripts, even though portions of the text of the
-#   Macro appear in them. The GNU General Public License (GPL) does govern
-#   all other use of the material that constitutes the Autoconf Macro.
-#
-#   This special exception to the GPL applies to versions of the Autoconf
-#   Macro released by the Autoconf Archive. When you make and distribute a
-#   modified version of the Autoconf Macro, you may extend this special
-#   exception to the GPL to apply to your modified version as well.
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
 
-#serial 4
+#serial 6
 
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 [AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF

build-aux/m4/ax_check_preproc_flag.m4

@@ -1,5 +1,5 @@
 # ===========================================================================
-#   http://www.gnu.org/software/autoconf-archive/ax_check_preproc_flag.html
+#  https://www.gnu.org/software/autoconf-archive/ax_check_preproc_flag.html
 # ===========================================================================
 #
 # SYNOPSIS
@@ -29,33 +29,12 @@
 #   Copyright (c) 2008 Guido U. Draheim <[email protected]>
 #   Copyright (c) 2011 Maarten Bosmans <[email protected]>
 #
-#   This program is free software: you can redistribute it and/or modify it
-#   under the terms of the GNU General Public License as published by the
-#   Free Software Foundation, either version 3 of the License, or (at your
-#   option) any later version.
-#
-#   This program is distributed in the hope that it will be useful, but
-#   WITHOUT ANY WARRANTY; without even the implied warranty of
-#   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
-#   Public License for more details.
-#
-#   You should have received a copy of the GNU General Public License along
-#   with this program. If not, see <http://www.gnu.org/licenses/>.
-#
-#   As a special exception, the respective Autoconf Macro's copyright owner
-#   gives unlimited permission to copy, distribute and modify the configure
-#   scripts that are the output of Autoconf when processing the Macro. You
-#   need not follow the terms of the GNU General Public License when using
-#   or distributing such scripts, even though portions of the text of the
-#   Macro appear in them. The GNU General Public License (GPL) does govern
-#   all other use of the material that constitutes the Autoconf Macro.
-#
-#   This special exception to the GPL applies to versions of the Autoconf
-#   Macro released by the Autoconf Archive. When you make and distribute a
-#   modified version of the Autoconf Macro, you may extend this special
-#   exception to the GPL to apply to your modified version as well.
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
 
-#serial 4
+#serial 6
 
 AC_DEFUN([AX_CHECK_PREPROC_FLAG],
 [AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF