Also rename `..._DEFAULT_BUILD_FILE_CONFIG` to `..._DEFAULT_BUILD_TYPE`.
These name changes make the variables meaningful for future use by other
generators.
Prior to this fix, CMAKE_NMC_DEFAULT_CONFIGS would inherit "all" from
the union of CMAKE_NMC_DEFAULT_BUILD_FILE_CONFIG and
CMAKE_NMC_CROSS_CONFIGS. This is inconsistent with the behavior of the
"all" target signifying CMAKE_NMC_CROSS_CONFIGS. Update "all" in
CMAKE_NMC_DEFAULT_CONFIGS to inherit only from CMAKE_NMC_CROSS_CONFIGS.
Fix an issue where CMAKE_NMC_DEFAULT_BUILD_FILE_CONFIG is specified
and CMAKE_NMC_CROSS_CONFIGS is not, which results in a false error
with CMAKE_NMC_DEFAULT_CONFIGS.
Many users will want to use the Ninja Multi-Config generator like a
traditional Visual Studio-style multi-config generator, which doesn't
mix configurations - custom commands are built using target executables
of the same configuration the command is for. We do not want to force
these people to generate an N*N build matrix when they only need N*1,
especially if they have lots of targets. Add a new variable,
CMAKE_NINJA_CROSS_CONFIG_ENABLE, to opt-in to the cross-config build
matrix.
Kyle Edwards