[libcxx] [llvm] [libc++][math][c++17] P0226R1 - Mathematical Special Functions: infra + `std::assoc_laguerre` (PR #205649)

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@@ -860,6 +864,14 @@ add_custom_target(cxx-test-depends
   COMMENT "Build dependencies required to run the libc++ test suite.")
 
 add_subdirectory(include)
+
+# Header-only Boost.Math (INTERFACE target `boost_math`). Lives outside the
+# libcxx source tree, so a binary dir must be supplied.
+add_subdirectory(
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PaulXiCao wrote:

Agreed on insulating from Boost's build. ExternalProject failed at configure time:
```
CMake Error at libcxx/src/CMakeLists.txt:180 (target_link_libraries):
  Target "boost_math" of type UTILITY may not be linked into another target.
  One may link only to INTERFACE, OBJECT, STATIC or SHARED libraries, or to
  executables with the ENABLE_EXPORTS property set.
```
Instead I've added `cmake/Modules/FindBoostMath.cmake` (similar implementation as FindLibcCommonUtils.cmake`).



https://github.com/llvm/llvm-project/pull/205649