Restore the Multithreading component

The documented Multithreading component disappeared together with the
optional threading backends, so requiring it when finding Ceres fails
although every build supports multithreading.

Always provide the component.

Change-Id: I5d6db6044a7f6a7596a11568910578ded8063055

cmake/CeresCompileOptionsToComponents.cmake

diff --git a/cmake/CeresCompileOptionsToComponents.cmake b/cmake/CeresCompileOptionsToComponents.cmake
index bbe39cc..26d9085 100644
--- a/cmake/CeresCompileOptionsToComponents.cmake
+++ b/cmake/CeresCompileOptionsToComponents.cmake
@@ -84,6 +84,8 @@
     CERES_NO_CUDSS "cuDSS;SparseLinearAlgebraLibrary")
   add_to_output_if_not_found(CURRENT_CERES_COMPILE_OPTIONS ${CERES_COMPONENTS_VAR}
     CERES_RESTRICT_SCHUR_SPECIALIZATION "SchurSpecializations")
+  # Multithreading is always available.
+  list(APPEND ${CERES_COMPONENTS_VAR} Multithreading)
   # Remove duplicates of SparseLinearAlgebraLibrary if multiple sparse backends
   # are present.
   list(REMOVE_DUPLICATES ${CERES_COMPONENTS_VAR})

docs/source/installation.rst

diff --git a/docs/source/installation.rst b/docs/source/installation.rst
index 8214e23..9511f5e 100644
--- a/docs/source/installation.rst
+++ b/docs/source/installation.rst
@@ -896,7 +896,8 @@
 #. ``SchurSpecializations``: Ceres built with Schur specializations
    (``WITH_SCHUR_SPECIALIZATIONS=ON``).
 
-#. ``Multithreading``: Ceres built with multithreading support.
+#. ``Multithreading``: Ceres built with multithreading support, which is
+   always available.
 
 To specify one/multiple Ceres components use the ``COMPONENTS`` argument to
 `find_package()