recent base master cannot set configure.compiler from the command line

[jeremyhu (Jeremy Huddleston Sequoia)]

I'm unable to set configure.compiler from the command line like I used to be able to:

~ $ sudo port -v -s build gnutls configure.compiler=macports-clang-3.7
--->  Computing dependencies for gnutls.
--->  Fetching distfiles for gnutls
--->  Verifying checksums for gnutls
--->  Checksumming gnutls-3.4.17.tar.xz
--->  Extracting gnutls
--->  Extracting gnutls-3.4.17.tar.xz
Executing:  cd "/opt/local/var/macports/build/_Users_jeremy_src_macports_macports-ports_devel_gnutls/gnutls/work" && /opt/local/bin/xz -dc '/opt/local/var/macports/distfiles/gnutls/gnutls-3.4.17.tar.xz' | /usr/bin/gnutar --no-same-owner -xf - 
--->  Applying patches to gnutls
--->  Applying patch-tests-pkcs11-pkcs11-mock.c.diff
Executing:  cd "/opt/local/var/macports/build/_Users_jeremy_src_macports_macports-ports_devel_gnutls/gnutls/work/gnutls-3.4.17" && /usr/bin/patch -p0 < '/Users/jeremy/src/macports/macports-ports/devel/gnutls/files/patch-tests-pkcs11-pkcs11-mock.c.diff'
patching file tests/pkcs11/pkcs11-mock.c
--->  Patching aes-ssse3-x86.s: /^#/d
--->  Patching aes-ssse3-x86_64.s: /^#/d
--->  Patching aesni-x86.s: /^#/d
--->  Patching aesni-x86_64.s: /^#/d
--->  Patching cpuid-x86.s: /^#/d
--->  Patching cpuid-x86_64.s: /^#/d
--->  Patching e_padlock-x86.s: /^#/d
--->  Patching e_padlock-x86_64.s: /^#/d
--->  Patching ghash-x86_64.s: /^#/d
--->  Patching sha1-ssse3-x86.s: /^#/d
--->  Patching sha1-ssse3-x86_64.s: /^#/d
--->  Patching sha256-ssse3-x86.s: /^#/d
--->  Patching sha512-ssse3-x86.s: /^#/d
--->  Patching sha512-ssse3-x86_64.s: /^#/d
--->  Configuring gnutls
Error: Failed to configure gnutls: Invalid value for configure.compiler: "macports-clang-3.7"
Error: See /opt/local/var/macports/logs/_Users_jeremy_src_macports_macports-ports_devel_gnutls/gnutls/main.log for details.
Error: Follow https://guide.macports.org/#project.tickets to report a bug.
Error: Processing of port gnutls failed

[ryandesign (Ryan Carsten Schmidt)]

I assume this is the same issue as #53113.