Upgrade macos CI to use boost 1.75.

Because of a bug in brew, the symlink `2to3` must be deleted before
upgrading python.

Boost 1.75 requires at least c++14.  We'll just use c++14 for all
compilation, not just macos.

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu, macos]
-        boost: ['1_60', '1_66', '1_74']
+        boost: ['1_60', '1_66', '1_75']
         geos: ['3.8.1', 'none']
         compiler: ['g++', 'g++-8', 'clang++']
         exclude:
@@ -23,7 +23,7 @@ jobs:
           - os: macos
             boost: '1_66'
           - os: ubuntu
-            boost: '1_74'
+            boost: '1_75'
           - os: macos
             compiler: 'g++-8'
         include:
@@ -68,16 +68,17 @@ jobs:
       if: matrix.os == 'macos'
       run: |
         brew update
+        rm /usr/local/bin/2to3
         brew upgrade python@3
         brew reinstall -s libtool
         brew unlink parallel
-        brew install boost@1.74 gtkmm moreutils gettext librsvg autoconf automake pkg-config
+        brew install boost@1.75 gtkmm moreutils gettext librsvg autoconf automake pkg-config
         brew upgrade wget
         hash -r
         ln -f -s `which glibtoolize` $HOME/.local/bin/libtoolize
         ln -f -s `which glibtool` $HOME/.local/bin/libtool
         hash -r
-        echo "BOOST_ROOT=/usr/local/opt/boost@1.74" >> $GITHUB_ENV
+        echo "BOOST_ROOT=/usr/local/opt/boost@1.75" >> $GITHUB_ENV
         echo "NUM_CPUS=$((`sysctl -n hw.logicalcpu` * 4))" >> $GITHUB_ENV
         echo "PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/opt/libffi/lib/pkgconfig" >> $GITHUB_ENV
         echo "CPPFLAGS_gerbv=-DQUARTZ" >> $GITHUB_ENV

configure.ac

@@ -21,7 +21,7 @@ AC_PROG_CC
 AC_PROG_LIBTOOL
 
 # Check for C++11
-AX_CXX_COMPILE_STDCXX_11
+AX_CXX_COMPILE_STDCXX_14
 
 # As reported here https://svn.boost.org/trac/boost/ticket/9240,
 # Boost requires -fext-numeric-literals when built with C++11

m4/ax_cxx_compile_stdcxx_14.m4

@@ -0,0 +1,34 @@
+# =============================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx_14.html
+# =============================================================================
+#
+# SYNOPSIS
+#
+#   AX_CXX_COMPILE_STDCXX_14([ext|noext], [mandatory|optional])
+#
+# DESCRIPTION
+#
+#   Check for baseline language coverage in the compiler for the C++14
+#   standard; if necessary, add switches to CXX and CXXCPP to enable
+#   support.
+#
+#   This macro is a convenience alias for calling the AX_CXX_COMPILE_STDCXX
+#   macro with the version set to C++14.  The two optional arguments are
+#   forwarded literally as the second and third argument respectively.
+#   Please see the documentation for the AX_CXX_COMPILE_STDCXX macro for
+#   more information.  If you want to use this macro, you also need to
+#   download the ax_cxx_compile_stdcxx.m4 file.
+#
+# LICENSE
+#
+#   Copyright (c) 2015 Moritz Klammler <moritz@klammler.eu>
+#
+#   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 5
+
+AX_REQUIRE_DEFINED([AX_CXX_COMPILE_STDCXX])
+AC_DEFUN([AX_CXX_COMPILE_STDCXX_14], [AX_CXX_COMPILE_STDCXX([14], [$1], [$2])])