Fix failures while building runtime library

Flang builds occasionally fails with errors like:
    F90-F-0004-Corrupt or Old Module file ../../include/__norm2.mod (.../flang/runtime/flang/norm2.F95: 1270)

Serialize the build of flang_shared
and flang_static to eliminate conflicts
with the same module files being
created/recreated in the common directory
${CMAKE_Fortran_MODULE_DIRECTORY}.

The build of each library is still parallelized.

Author: gklimowicz

runtime/flang/CMakeLists.txt

@@ -495,6 +495,17 @@ add_flang_library(flang_shared
   ${SHARED_SOURCES}
   )
 set_property(TARGET flang_shared PROPERTY OUTPUT_NAME flang)
+
+#
+# Seralize the building of flang_shared and flang_static to eliminate
+# conflicts with the same module files from the shared and static builds
+# being created/recreated in the common directory
+# ${CMAKE_Fortran_MODULE_DIRECTORY}.
+#
+# Note: building of each library is still parallelized.
+#
+add_dependencies(flang_shared flang_static)
+
 target_link_libraries(flang_shared ${CMAKE_BINARY_DIR}/${CMAKE_CFG_INTDIR}/lib/libflangrti.so)
 # Resolve symbols against libm and librt
 target_link_libraries(flang_shared m rt)