Commit 2f4d8cf8 fixed the following error:

  Generating phar.php
  .../parts/apache-php__compile__/php-7.3.6/sapi/cli/php: error while loading shared libraries: libzstd.so.1: cannot open shared object file: No such file or directory
  Makefile:420: recipe for target 'ext/phar/phar.php' failed

and only -rpath is needed.

apache-php does not actually depend on zstd (hence the removal of the 'extends'
line) but there's something not smart:

  $ ldd .../parts/apache-php__compile__/php-7.3.6/sapi/cli/php
        linux-vdso.so.1 (0x00007ffe29f86000)
        libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007f625e64a000)
        libcrypto.so.1.1 => .../shared/openssl/.../lib/libcrypto.so.1.1 (0x00007f625e35e000)
        libssl.so.1.1 => .../shared/openssl/.../lib/libssl.so.1.1 (0x00007f625ea05000)
        libzip.so.5 => .../shared/libzip/.../lib/libzip.so.5 (0x00007f625e9ea000)
        libz.so.1 => .../shared/zlib/.../lib/libz.so.1 (0x00007f625e142000)
        libargon2.so.1 => /srv/slapgrid/slappart10/srv/runner/software/.../parts/argon2/lib/x86_64-linux-gnu/libargon2.so.1 (0x00007f625e9de000)
        librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f625df3a000)
        libldap-2.4.so.2 => .../shared/openldap/.../lib/libldap-2.4.so.2 (0x00007f625e991000)
        libsasl2.so.3 => .../shared/cyrus-sasl/.../lib/libsasl2.so.3 (0x00007f625e973000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f625dd36000)
        liblber-2.4.so.2 => .../shared/openldap/.../lib/liblber-2.4.so.2 (0x00007f625e963000)
        libresolv.so.2 => /lib/x86_64-linux-gnu/libresolv.so.2 (0x00007f625db1f000)
        libstdc++.so.6 => .../shared/gcc-8.4/.../lib/../lib64/libstdc++.so.6 (0x00007f625d994000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f625d690000)
        libintl.so.8 => .../shared/gettext/.../lib/libintl.so.8 (0x00007f625d485000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f625d0e6000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f625e882000)
        libjpeg.so.9 => .../shared/libjpeg/.../lib/libjpeg.so.9 (0x00007f625e924000)
        libnsl.so.1 => /lib/x86_64-linux-gnu/libnsl.so.1 (0x00007f625cece000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f625ccb1000)
        libcurl.so.4 => .../shared/curl/.../lib/libcurl.so.4 (0x00007f625e8ac000)
        libnghttp2.so.14 => .../shared/nghttp2/.../lib/libnghttp2.so.14 (0x00007f625cc87000)
        libzstd.so.1 => not found
        libfreetype.so.6 => .../shared/freetype/.../lib/libfreetype.so.6 (0x00007f625cbe8000)
        libbz2.so.1.0 => .../shared/bzip2/.../lib/libbz2.so.1.0 (0x00007f625cbd5000)
        libpng16.so.16 => .../shared/libpng/.../lib/libpng16.so.16 (0x00007f625cba0000)
        libicui18n.so.58 => .../shared/icu/.../lib/libicui18n.so.58 (0x00007f625c914000)
        libicuuc.so.58 => .../shared/icu/.../lib/libicuuc.so.58 (0x00007f625c762000)
        libicudata.so.58 => .../shared/icu/.../lib/libicudata.so.58 (0x00007f625ae60000)
        libicuio.so.58 => .../shared/icu/.../lib/libicuio.so.58 (0x00007f625ae50000)
        libxml2.so.2 => .../shared/libxml2/.../lib/libxml2.so.2 (0x00007f625aaef000)
        libgcc_s.so.1 => .../shared/gcc-8.4/.../lib/../lib64/libgcc_s.so.1 (0x00007f625aad5000)
        libzstd.so.1 => .../shared/zstd/.../lib/libzstd.so.1 (0x00007f625a9fb000)

(look at the 2 'libzstd.so.1 => ' lines above)

  $ grep zstd .../shared/curl/.../lib/pkgconfig/libcurl.pc
  supported_features="..."
  Libs.private: -lnghttp2 -lssl -lcrypto -lssl -lcrypto -lzstd -lzstd -lz -pthread

  $ ldd .../shared/curl/.../lib/libcurl.so.4
        linux-vdso.so.1 (0x00007ffc42b15000)
        libnghttp2.so.14 => .../shared/nghttp2/.../lib/libnghttp2.so.14 (0x00007f54315d5000)
        libssl.so.1.1 => .../shared/openssl/.../lib/libssl.so.1.1 (0x00007f543153f000)
        libcrypto.so.1.1 => .../shared/openssl/.../lib/libcrypto.so.1.1 (0x00007f5431168000)
        libzstd.so.1 => .../shared/zstd/.../lib/libzstd.so.1 (0x00007f543108e000)
        libz.so.1 => .../shared/zlib/.../lib/libz.so.1 (0x00007f5430e72000)
        libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f5430c55000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f54308b6000)
        libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f54306b2000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f5431454000)