Define _REENTRANT for SUNOS (#104527)

This is required to make errno a thread-local variable. See the manpages for errno(3c) and threads(7).

Author: AustinWise

eng/native/configurecompiler.cmake

@@ -304,7 +304,7 @@ elseif(CLR_CMAKE_HOST_SUNOS)
   add_compile_options($<$<COMPILE_LANGUAGE:ASM>:-Wa,--noexecstack>)
   set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fstack-protector")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstack-protector")
-  add_definitions(-D__EXTENSIONS__ -D_XPG4_2 -D_POSIX_PTHREAD_SEMANTICS)
+  add_definitions(-D__EXTENSIONS__ -D_XPG4_2 -D_POSIX_PTHREAD_SEMANTICS -D_REENTRANT)
 elseif(CLR_CMAKE_HOST_OSX AND NOT CLR_CMAKE_HOST_MACCATALYST AND NOT CLR_CMAKE_HOST_IOS AND NOT CLR_CMAKE_HOST_TVOS)
   add_definitions(-D_XOPEN_SOURCE)