Fix portable mac shared_library ld flags (#9149)

Summary:
Move the 'macosx-version-min' arg to the front of PLATFORM_SHARED_LDFLAGS so that it doesn't get concatenated with the library name. Fixes https://github.com/facebook/rocksdb/issues/9146

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9149

Reviewed By: mrambacher

Differential Revision: D32396101

Pulled By: pdillinger

fbshipit-source-id: aefcf53384e64d399049f158779acc3a4e54a8fe

build_tools/build_detect_platform

@@ -675,7 +675,8 @@ else
     # For portability compile for macOS 10.12 (2016) or newer
     COMMON_FLAGS="$COMMON_FLAGS -mmacosx-version-min=10.12"
     PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -mmacosx-version-min=10.12"
-    PLATFORM_SHARED_LDFLAGS="$PLATFORM_SHARED_LDFLAGS -mmacosx-version-min=10.12"
+    # -mmacosx-version-min must come first here.
+    PLATFORM_SHARED_LDFLAGS="-mmacosx-version-min=10.12 $PLATFORM_SHARED_LDFLAGS"
     PLATFORM_CMAKE_FLAGS="-DCMAKE_OSX_DEPLOYMENT_TARGET=10.12"
     JAVA_STATIC_DEPS_COMMON_FLAGS="-mmacosx-version-min=10.12"
     JAVA_STATIC_DEPS_LDFLAGS="$JAVA_STATIC_DEPS_COMMON_FLAGS"