cmake: drop _BSD_SOURCE macro usage

autotools does not use features.h nor _BSD_SOURCE. As this macro
triggers warnings since glibc 2.20, remove it. It should not have
functional differences.

Signed-off-by: Peter Wu <peter@lekensteyn.nl>

lib/CMakeLists.txt

@@ -48,25 +48,6 @@ endif()
 # )
 # ENDIF(NOT HAVE_STRTOLL AND NOT HAVE__STRTOI64)
 
-if(HAVE_FEATURES_H)
-  set_source_files_properties(
-    cookie.c
-    easy.c
-    formdata.c
-    getenv.c
-    nonblock.c
-    hash.c
-    http.c
-    if2ip.c
-    mprintf.c
-    multi.c
-    sendf.c
-    telnet.c
-    transfer.c
-    url.c
-    COMPILE_FLAGS -D_BSD_SOURCE)
-endif(HAVE_FEATURES_H)
-
 
 # The rest of the build