Rebuild for CMake Ninja generator change proposal

Signed-off-by: Cristian Le <git@lecris.dev>

vtk.spec

@@ -1,5 +1,3 @@
-%undefine __cmake_in_source_build
-
 # State Nov 11 2020, LTO causes
 # TestXMLHyperTreeGridIO.cxx.o (symbol from plugin): undefined reference to symbol
 # '_ZZNSt8__detail18__to_chars_10_implIjEEvPcjT_E8__digits@@LLVM_11'
@@ -78,7 +76,10 @@ URL: https://vtk.org/
 BuildRequires:  cmake
 # Allow for testing with different cmake generators.
 # make still seems to be faster than ninja, but has failed at times.
-%global cmake_gen %{nil}
+# Note: there is a transitive `make` dependence that interferes with
+#  automatically switching to Ninja generator, add explicit dependence
+#  when testing a new generator.
+%global _cmake_generator "Unix Makefiles"
 BuildRequires:  gcc-c++
 %if %{with java}
 BuildRequires: java-devel
@@ -618,7 +619,7 @@ export JAVA_TOOL_OPTIONS=-Xmx2048m
 %endif
 %endif
 
-%cmake %{cmake_gen} \
+%cmake \
  %{vtk_cmake_options} \
  -DVTK_BUILD_DOCUMENTATION:BOOL=ON \
  -DVTK_BUILD_EXAMPLES:BOOL=ON \
@@ -633,7 +634,7 @@ for mpi in %{mpi_list}
 do
   module load mpi/$mpi-%{_arch}
   #CMAKE_INSTALL_LIBDIR -> ARCHIVE_DESTINATION must not be an absolute path
-  %cmake %{cmake_gen} \
+  %cmake \
    %{vtk_cmake_options} \
    -DCMAKE_PREFIX_PATH:PATH=$MPI_HOME \
    -DCMAKE_INSTALL_PREFIX:PATH=$MPI_HOME \