we use _GNU_SOURCE for all of uClibc now

libm/Makefile.in

@@ -32,7 +32,6 @@ libm_FULL_NAME := libm-$(MAJOR_VERSION).$(MINOR_VERSION).$(SUBLEVEL).so
 libm_ARCH_DIR:=$(top_srcdir)libm/$(TARGET_ARCH)
 libm_ARCH_OUT:=$(top_builddir)libm/$(TARGET_ARCH)
 
-CFLAGS-libm/$(TARGET_ARCH)/fpu/ += -D_GNU_SOURCE
 libm_ARCH_fpu_DIR:=$(libm_ARCH_DIR)/fpu
 libm_ARCH_fpu_OUT:=$(libm_ARCH_OUT)/fpu