icu @76.1 fails to build with gcc on 10.15

[barracuda156]

/opt/local/bin/g++-mp-14 -DU_ALL_IMPLEMENTATION -DU_ATTRIBUTE_DEPRECATED= -DU_COMMON_IMPLEMENTATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -DU_HAVE_STRTOD_L=1 -DU_HAVE_XLOCALE_H=1 -I. "-DDEFAULT_ICU_PLUGINS=\"/opt/local/lib/icu\" " -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++17 -fvisibility=hidden -fno-common -c -MMD -MT "putil.d putil.o putil.ao" -o putil.ao putil.cpp
In file included from /opt/local/include/gcc14/c++/stdlib.h:36,
                 from uassert.h:23,
                 from putil.cpp:62:
/opt/local/include/gcc14/c++/cstdlib:136:11: error: 'aligned_alloc' has not been declared in '::'
  136 |   using ::aligned_alloc;
      |           ^~~~~~~~~~~~~
/opt/local/bin/g++-mp-14 -DU_ALL_IMPLEMENTATION -DU_ATTRIBUTE_DEPRECATED= -DU_COMMON_IMPLEMENTATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -DU_HAVE_STRTOD_L=1 -DU_HAVE_XLOCALE_H=1 -I. "-DDEFAULT_ICU_PLUGINS=\"/opt/local/lib/icu\" " -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++17 -fvisibility=hidden -fno-common -c -MMD -MT "rbbi.d rbbi.o rbbi.ao" -o rbbi.ao rbbi.cpp
In file included from /opt/local/include/gcc14/c++/bits/chrono.h:40,
                 from /opt/local/include/gcc14/c++/condition_variable:40,
                 from umutex.h:24,
                 from putil.cpp:63:
/opt/local/include/gcc14/c++/ctime:80:11: error: 'timespec_get' has not been declared in '::'
   80 |   using ::timespec_get;
      |           ^~~~~~~~~~~~
/opt/local/bin/g++-mp-14 -DU_ALL_IMPLEMENTATION -DU_ATTRIBUTE_DEPRECATED= -DU_COMMON_IMPLEMENTATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -DU_HAVE_STRTOD_L=1 -DU_HAVE_XLOCALE_H=1 -I. "-DDEFAULT_ICU_PLUGINS=\"/opt/local/lib/icu\" " -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++17 -fvisibility=hidden -fno-common -c -MMD -MT "rbbi_cache.d rbbi_cache.o rbbi_cache.ao" -o rbbi_cache.ao rbbi_cache.cpp
/opt/local/bin/g++-mp-14 -DU_ALL_IMPLEMENTATION -DU_ATTRIBUTE_DEPRECATED= -DU_COMMON_IMPLEMENTATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -DU_HAVE_STRTOD_L=1 -DU_HAVE_XLOCALE_H=1 -I. "-DDEFAULT_ICU_PLUGINS=\"/opt/local/lib/icu\" " -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++17 -fvisibility=hidden -fno-common -c -MMD -MT "rbbidata.d rbbidata.o rbbidata.ao" -o rbbidata.ao rbbidata.cpp
/opt/local/bin/g++-mp-14 -DU_ALL_IMPLEMENTATION -DU_ATTRIBUTE_DEPRECATED= -DU_COMMON_IMPLEMENTATION -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -DU_HAVE_STRTOD_L=1 -DU_HAVE_XLOCALE_H=1 -I. "-DDEFAULT_ICU_PLUGINS=\"/opt/local/lib/icu\" " -pipe -Os -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -arch x86_64 -W -Wall -pedantic -Wpointer-arith -Wwrite-strings -Wno-long-long -std=c++17 -fvisibility=hidden -fno-common -c -MMD -MT "rbbinode.d rbbinode.o rbbinode.ao" -o rbbinode.ao rbbinode.cpp
gnumake[1]: *** [putil.ao] Error 1
gnumake[1]: *** Waiting for unfinished jobs....
gnumake[1]: Leaving directory `/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_macports_release_tarballs_ports_devel_icu/icu/work/icu/source/common'
gnumake: *** [all-recursive] Error 2

This is against libstdc++, but it should not fail regardless (it works on powerpc!).

[ryandesign (Ryan Carsten Schmidt)]

Don't use gcc or libstdc++ on 10.15. Use clang and libc++.

[barracuda156]

Replying to ryandesign:

Don't use gcc or libstdc++ on 10.15. Use clang and libc++.

It is useful for testing and development to have libstdc++-based system.

Here is a provisional fix anyway: ​https://github.com/unicode-org/icu/pull/3465