[llvm] [LLVM][Cygwin] Fix shared library name (PR #136599)

[via llvm-commits]

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@@ -64,7 +64,11 @@ set(LLVM_CFLAGS "${LLVM_C_STD_FLAG} ${LLVM_DEFINITIONS}")
 set(LLVM_CXXFLAGS "${CMAKE_CXX${CMAKE_CXX_STANDARD}_STANDARD_COMPILE_OPTION} ${LLVM_CXX_STDLIB_FLAG} ${COMPILE_FLAGS} ${LLVM_DEFINITIONS}")
 set(LLVM_BUILD_SYSTEM cmake)
 set(LLVM_HAS_RTTI ${LLVM_CONFIG_HAS_RTTI})
-set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}")
+if(CYGWIN)
+  set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}${LLVM_VERSION_SUFFIX}")
+else()
+  set(LLVM_DYLIB_VERSION "${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}")
+endif()
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jeremyd2019 wrote:

> I think that trying to mimic the mingw behaviour here probably is the most reasonable thing to do. (The broken symlinks is something that doesn't really show up in mingw cases I think?)

The broken symlinks are in an `if (UNIX)` block, which I now patched to be `if (UNIX AND NOT CYGWIN)`, and also patched the function making the symlinks to know shared libs live in `BINDIR` on `CYGWIN` as well as `WIN32`.

https://github.com/llvm/llvm-project/pull/136599