Changes (by Samuel Tootle):

When compiling ET_2020_05, I obtained linking errors against LAPACK and BLAS even though these were set to BUILD in the optionslist.

After further review, the issue turned out to be that bindings/Configuration/Capabilities/make.BLAS.defn and make.LAPACK.defn both had the libraries listed in LAPACK_DIR/lib and BLAS_DIR/lib respectively, however, the configure.sh scripts for both libs copies the static library to LAPACK_DIR and BLAS_DIR respectively. For instance:
ExternalLibraries/LAPACK/configure.sh

        echo "LAPACK: Installing..."
        cp liblapack.a ${LAPACK_DIR}
        popd

Changing the LAPACK_LIB_DIRS and BLAS_LIB_DIRS to the base directory where the library is located results in correct linking.

For instance, Old make.BLAS.defn:

INC_DIRS +=  $(BLAS_INC_DIRS)
INC_DIRS_F +=  $(BLAS_INC_DIRS)
BLAS_DIR      = /<directory_tree to FreeTHC>/FreeTHC2020/Cactus/configs/thc_intel/scratch/external/BLAS
BLAS_INC_DIRS =
BLAS_LIB_DIRS = /<directory_tree to FreeTHC>/FreeTHC2020/Cactus/configs/thc_intel/scratch/external/BLAS/lib
BLAS_LIBS     = blas
HAVE_CAPABILITY_BLAS = 1

New:

NC_DIRS +=  $(BLAS_INC_DIRS)
--

Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2464/linking-errors-with-et_2020_05-with-etk