[OpenMP] Allow setting OPENMP_INSTALL_LIBDIR (#77533)

The comment indicate that it should be possible, but as long as it
wasn't a cache variable, the cmake script overwrote whatever variable
the user had set.

Author: mstorsjo

openmp/CMakeLists.txt

@@ -29,7 +29,8 @@ if (OPENMP_STANDALONE_BUILD)
   set(OPENMP_LIBDIR_SUFFIX "" CACHE STRING
     "Suffix of lib installation directory, e.g. 64 => lib64")
   # Do not use OPENMP_LIBDIR_SUFFIX directly, use OPENMP_INSTALL_LIBDIR.
-  set(OPENMP_INSTALL_LIBDIR "lib${OPENMP_LIBDIR_SUFFIX}")
+  set(OPENMP_INSTALL_LIBDIR "lib${OPENMP_LIBDIR_SUFFIX}" CACHE STRING
+      "Path where built OpenMP libraries should be installed.")
 
   # Group test settings.
   set(OPENMP_TEST_C_COMPILER ${CMAKE_C_COMPILER} CACHE STRING
@@ -46,7 +47,8 @@ if (OPENMP_STANDALONE_BUILD)
 else()
   set(OPENMP_ENABLE_WERROR ${LLVM_ENABLE_WERROR})
   # If building in tree, we honor the same install suffix LLVM uses.
-  set(OPENMP_INSTALL_LIBDIR "lib${LLVM_LIBDIR_SUFFIX}")
+  set(OPENMP_INSTALL_LIBDIR "lib${LLVM_LIBDIR_SUFFIX}" CACHE STRING
+      "Path where built OpenMP libraries should be installed.")
 
   if (NOT MSVC)
     set(OPENMP_TEST_C_COMPILER ${LLVM_RUNTIME_OUTPUT_INTDIR}/clang)