[LLVM][Cygwin] Define _GNU_SOURCE on Cygwin as well.

Without it, certain functions such as dladdr are not make available by
the headers.

Signed-off-by: Jeremy Drake <github@jdrake.com>

Author: jeremyd2019

llvm/cmake/config-ix.cmake

@@ -86,7 +86,7 @@ endif()
 # Keep this at the top to make sure we don't add _GNU_SOURCE dependent checks
 # before adding it.
 check_symbol_exists(__GLIBC__ stdio.h LLVM_USING_GLIBC)
-if(LLVM_USING_GLIBC)
+if(LLVM_USING_GLIBC OR CYGWIN)
   add_compile_definitions(_GNU_SOURCE)
   list(APPEND CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")