CMake generation for SLANG_LIB_TYPE = STATIC broken since 5d8cf47

[James2022-rgb]

CMake generation for SLANG_LIB_TYPE = STATIC fails with the following error message:

CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "core" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "prelude" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "compiler-core" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "slang-capability-defs" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "slang-capability-lookup" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "slang-reflect-headers" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "slang-lookup-tables" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "SPIRV-Headers" that is not in any export set.
CMake Error: install(EXPORT "SlangExportTarget" ...) includes target "slang" which requires target "slang-common-objects" that is not in any export set.

git bisect'ing seems to suggest that this breakage was introduced in the commit 5d8cf47 (#5674).

I tried to see if I could help fix it, only to realize I was not versed enough in CMake.