[CMake] Fix install failure with CMAKE_SLANG_FRONTEND_ENABLED enabled (…

…#7437)

Slang compiler relies on the third-party libraries like unordered_dense and fmt
library. The fmt library provides two ways to integrated it:
  1.Headers-only
  2.Seperately compiled
The main purpose of this commit is to avoid installation failure of CIRCT project due
to finding fmt header file in wrong path which is in circt `include` directory when
CMake_slang_Frontend_enabled is turned on. We hope to not install header files coming
from fmt library.

CMakeLists.txt

@@ -591,6 +591,13 @@ if(CIRCT_SLANG_FRONTEND_ENABLED)
     # harder than it ought to be.
     set_property(
       GLOBAL APPEND PROPERTY CIRCT_EXPORTS slang_slang unordered_dense fmt)
+
+    # Disable the installation of headers coming from third-party libraries. We
+    # won't use those APIs directly. Just make them static libraries for the sake
+    # of running slang normally.
+    set_target_properties(fmt PROPERTIES PUBLIC_HEADER "")
+    set_target_properties(unordered_dense PROPERTIES PUBLIC_HEADER "")
+
     install(TARGETS slang_slang unordered_dense fmt EXPORT CIRCTTargets)
   else()
     find_package(slang 3.0 REQUIRED)