This does not work...

+ /usr/lib/rpm/cmake-configure /usr/bin/cmake . x86_64-redhat-linux-gnu -DCMAKE_C_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_CXX_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_Fortran_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DCMAKE_INSTALL_PREFIX:PATH=/usr -DINCLUDE_INSTALL_DIR:PATH=/usr/include -DLIB_INSTALL_DIR:PATH=/usr/lib64 -DSYSCONF_INSTALL_DIR:PATH=/etc -DSHARE_INSTALL_PREFIX:PATH=/usr/share -DLIB_SUFFIX=64 -DBUILD_SHARED_LIBS:BOOL=ON -DCastXML_INSTALL_DOC_DIR:STRING=share/doc/castxml -DCastXML_INSTALL_MAN_DIR:STRING=share/man -DCLANG_RESOURCE_DIR:PATH=/usr/lib64/clang/9.0.1/include/.. -DLLVM_LINK_LLVM_DYLIB:BOOL=ON -DCLANG_LINK_CLANG_DYLIB:BOOL=ON -DBUILD_TESTING:BOOL=ON -DSPHINX_MAN:BOOL=ON
CMake Error: The source directory "/builddir/build/BUILD/CastXML-0.3.4/x86_64-redhat-linux-gnu" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
error: Bad exit status from /var/tmp/rpm-tmp.WqUIAk (%build)

With the following change:

$ diff -u /var/lib/mock/epel-8-x86_64/root/usr/lib/rpm/cmake-configure{~,}
--- /var/lib/mock/epel-8-x86_64/root/usr/lib/rpm/cmake-configure~   2020-07-30 00:55:54.000000000 +0200
+++ /var/lib/mock/epel-8-x86_64/root/usr/lib/rpm/cmake-configure    2020-07-30 20:43:21.668622047 +0200
@@ -9,5 +9,6 @@

 # Do the build
 mkdir -p "${__cmake_builddir}"
-"${__cmake}" ${__cmake_flags} "${__cmake_srcdir}" "${__cmake_builddir}"
+cd "${__cmake_builddir}"
+"${__cmake}" ${__cmake_flags} "../${__cmake_srcdir}"
 exit $?

it works as expected:

+ /usr/lib/rpm/cmake-configure /usr/bin/cmake . x86_64-redhat-linux-gnu -DCMAKE_C_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_CXX_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_Fortran_FLAGS_RELEASE:STRING=-DNDEBUG -DCMAKE_VERBOSE_MAKEFILE:BOOL=ON -DCMAKE_INSTALL_PREFIX:PATH=/usr -DINCLUDE_INSTALL_DIR:PATH=/usr/include -DLIB_INSTALL_DIR:PATH=/usr/lib64 -DSYSCONF_INSTALL_DIR:PATH=/etc -DSHARE_INSTALL_PREFIX:PATH=/usr/share -DLIB_SUFFIX=64 -DBUILD_SHARED_LIBS:BOOL=ON -DCastXML_INSTALL_DOC_DIR:STRING=share/doc/castxml -DCastXML_INSTALL_MAN_DIR:STRING=share/man -DCLANG_RESOURCE_DIR:PATH=/usr/lib64/clang/9.0.1/include/.. -DLLVM_LINK_LLVM_DYLIB:BOOL=ON -DCLANG_LINK_CLANG_DYLIB:BOOL=ON -DBUILD_TESTING:BOOL=ON -DSPHINX_MAN:BOOL=ON
-- The C compiler identification is GNU 8.3.1
-- The CXX compiler identification is GNU 8.3.1
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_CXX_FLAGS_RELEASE
    CMAKE_C_FLAGS_RELEASE
    CMAKE_Fortran_FLAGS_RELEASE
    INCLUDE_INSTALL_DIR
    LIB_INSTALL_DIR
    LIB_SUFFIX
    SHARE_INSTALL_PREFIX
    SYSCONF_INSTALL_DIR


-- Build files have been written to: /builddir/build/BUILD/CastXML-0.3.4/x86_64-redhat-linux-gnu
+ /usr/lib/rpm/cmake-build x86_64-redhat-linux-gnu -j2