CMake: Fix build with OpenBSD #711
57be074f6f300fcacd361f52dea62874c7214084

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -455,7 +455,7 @@ if(WIN32)
     set(MZ_LIBBSD OFF)
     set(MZ_ICONV OFF)
 else()
-    list(APPEND STDLIB_DEF -D_POSIX_C_SOURCE=200809L)
+    list(APPEND STDLIB_DEF -D_POSIX_C_SOURCE=200809L -D_BSD_SOURCE)
     list(APPEND MINIZIP_SRC mz_os_posix.c mz_strm_os_posix.c)
 
     if(MZ_PKCRYPT OR MZ_WZAES)