build : move _WIN32_WINNT definition to headers (llama/17736)

Previously, cmake was forcing `_WIN32_WINNT=0x0A00` for MinGW builds,
This caused "macro redefined" warnings with toolchains that define the version.

This also removes the `GGML_WIN_VER` variable as it is no longer needed.

Signed-off-by: Adrien Gallouët <angt@huggingface.co>

ggml/CMakeLists.txt

@@ -175,11 +175,6 @@ option(GGML_CPU_ALL_VARIANTS "ggml: build all variants of the CPU backend (requi
 set(GGML_CPU_ARM_ARCH        "" CACHE STRING "ggml: CPU architecture for ARM")
 set(GGML_CPU_POWERPC_CPUTYPE "" CACHE STRING "ggml: CPU type for PowerPC")
 
-
-if (MINGW)
-    set(GGML_WIN_VER "0xA00" CACHE STRING   "ggml: Windows version")
-endif()
-
 # ggml core
 set(GGML_SCHED_MAX_COPIES  "4" CACHE STRING "ggml: max input copies for pipeline parallelism")
 option(GGML_CPU                             "ggml: enable CPU backend"                        ON)

ggml/include/ggml.h

@@ -204,6 +204,10 @@
 #    define GGML_ATTRIBUTE_FORMAT(...) __attribute__((format(printf, __VA_ARGS__)))
 #endif
 
+#if defined(_WIN32) && !defined(_WIN32_WINNT)
+#    define _WIN32_WINNT 0x0A00
+#endif
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>

ggml/src/CMakeLists.txt

@@ -127,10 +127,6 @@ if (NOT MSVC)
     endif()
 endif()
 
-if (MINGW)
-    add_compile_definitions(_WIN32_WINNT=${GGML_WIN_VER})
-endif()
-
 #
 # POSIX conformance
 #