表題の通りです。
何卒ご教示のほどよろしくお願いいたします。

環境


OS	macOS Big Sur 11.4
CPU	2.6 GHz 6コアIntel Core i7
Homebrew	3.1.11-18-ged0ed9f
phpenv	rbenv 1.1.2-61-g585ed84

試したこと

icu4cのバージョンが問題であると判断し、64, 65, 66, 67, 69を試すもいずれもエラー
PHP7.2系のインストールは同様の手順で可能だった

コマンドとエラー内容

bash
1% export CFLAGS="-Wno-error=implicit-function-declaration -DU_DEFINE_FALSE_AND_TRUE=1"  
2
3% export CXXFLAGS="-Wno-error=implicit-function-declaration -DU_DEFINE_FALSE_AND_TRUE=1"
4
5% export PHP_BUILD_CONFIGURE_OPTS="--with-gd \
6 --enable-intl --enable-sockets --enable-exif --with-zlib \
7 --with-zlib-dir=$(brew --prefix zlib) --with-bz2 --with-bz2=$(brew --prefix bzip2) \
8 --with-iconv=$(brew --prefix libiconv) --with-kerberos \
9 --with-openssl=$(brew --prefix openssl) --enable-soap --enable-xmlreader --with-xsl \
10 --enable-ftp --enable-cgi \
11 --with-libedit=$(brew --prefix libedit) --with-tidy --with-xmlrpc --enable-sysvsem \
12 --enable-sysvshm --enable-shmop --with-mysqli=mysqlnd --with-pdo-mysql=mysqlnd \
13 --with-pdo-sqlite --enable-pcntl --with-readline --enable-mbstring --disable-debug \
14 --enable-fpm --enable-bcmath --enable-phpdbg --with-pear --with-curl=$(brew --prefix curl) \
15 --with-libcurl=$(brew --prefix curl) --with-libxml-dir=$(brew --prefix libxml2)"  
16
17% phpenv install 5.6.40   
18[Info]: Loaded extension plugin
19[Info]: Loaded apc Plugin.
20[Info]: Loaded composer Plugin.
21[Info]: Loaded github Plugin.
22[Info]: Loaded uprofiler Plugin.
23[Info]: Loaded xdebug Plugin.
24[Info]: Loaded xhprof Plugin.
25[Info]: Loaded zendopcache Plugin.
26[Info]: php.ini-production gets used as php.ini
27[Info]: Building 5.6.40 into /Users/[user_name]/.phpenv/versions/5.6.40
28[Downloading]: https://secure.php.net/distributions/php-5.6.40.tar.bz2
29[Info]: Applying patches: /Users/[user_name]/.phpenv/plugins/php-build/bin/../share/php-build/patches/php-5.6-support-openssl-1.1.0.patch
30[Preparing]: /var/tmp/php-build/source/5.6.40
31[Compiling]: /var/tmp/php-build/source/5.6.40
32
33-----------------
34|  BUILD ERROR  |
35-----------------
36
37Here are the last 10 lines from the log:
38
39-----------------------------------------
40        ((StringEnumeration*)iter->data)->reset(INTLITERATOR_ERROR_CODE(ii));
41          ^
42/var/tmp/php-build/source/5.6.40/ext/intl/common/common_enum.cpp:116:22: error: expected expression
43        ((StringEnumeration*)iter->data)->reset(INTLITERATOR_ERROR_CODE(ii));
44                            ^
45fatal error: too many errors emitted, stopping now [-ferror-limit=]
4615 warnings and 20 errors generated.
47make: *** [ext/intl/common/common_enum.lo] Error 1
4828 warnings and 20 errors generated.
49make: *** [ext/intl/common/common_date.lo] Error 1
50-----------------------------------------
51
52The full Log is available at '/tmp/php-build.5.6.40.20210609180254.log'.
53[Warn]: Aborting build.