diff --git a/coreconf/FreeBSD.mk b/coreconf/FreeBSD.mk
--- coreconf/FreeBSD.mk
+++ coreconf/FreeBSD.mk
@@ -21,7 +21,7 @@ ifeq ($(CPU_ARCH),amd64)
 CPU_ARCH		= x86_64
 endif
 
-OS_CFLAGS		= $(DSO_CFLAGS) -ansi -Wall -Wno-switch -DFREEBSD -DHAVE_STRERROR -DHAVE_BSD_FLOCK
+OS_CFLAGS		= $(DSO_CFLAGS) -Wall -Wno-switch -DFREEBSD -DHAVE_STRERROR -DHAVE_BSD_FLOCK
 
 DSO_CFLAGS		= -fPIC
 DSO_LDOPTS		= -shared -Wl,-soname -Wl,$(notdir $@)
diff --git a/coreconf/NetBSD.mk b/coreconf/NetBSD.mk
--- coreconf/NetBSD.mk
+++ coreconf/NetBSD.mk
@@ -26,7 +26,7 @@ else
 DLL_SUFFIX		= so.1.0
 endif
 
-OS_CFLAGS		= $(DSO_CFLAGS) $(OS_REL_CFLAGS) -ansi -Wall -Wno-switch -pipe -DNETBSD -Dunix -DHAVE_STRERROR -DHAVE_BSD_FLOCK
+OS_CFLAGS		= $(DSO_CFLAGS) $(OS_REL_CFLAGS) -Wall -Wno-switch -pipe -DNETBSD -Dunix -DHAVE_STRERROR -DHAVE_BSD_FLOCK
 
 OS_LIBS			= -lcompat
 
diff --git a/coreconf/OpenBSD.mk b/coreconf/OpenBSD.mk
--- coreconf/OpenBSD.mk
+++ coreconf/OpenBSD.mk
@@ -26,7 +26,7 @@ endif
 
 DLL_SUFFIX		= so.1.0
 
-OS_CFLAGS		= $(DSO_CFLAGS) $(OS_REL_CFLAGS) -ansi -Wall -Wno-switch -pipe -DOPENBSD
+OS_CFLAGS		= $(DSO_CFLAGS) $(OS_REL_CFLAGS) -Wall -Wno-switch -pipe -DOPENBSD
 
 OS_LIBS			= 
 
diff --git a/coreconf/Werror.mk b/coreconf/Werror.mk
--- coreconf/Werror.mk
+++ coreconf/Werror.mk
@@ -5,6 +5,15 @@
 
 # This sets WARNING_CFLAGS for gcc-like compilers.
 
+ifndef CC_IS_CLANG
+  CC_IS_CLANG := $(and $(findstring clang, $(shell $(CC) --version 2>&1)), 1)
+  # Clang claims GCC 4.2.1 compatibility, see GCC_VERSION
+  CC_IS_GCC = 1
+  # Export CC_IS_CLANG and CC_IS_GCC to save a shell invocation when recursing.
+  export CC_IS_CLANG
+  export CC_IS_GCC
+endif
+
 ifndef CC_IS_GCC
   CC_IS_GCC := $(shell $(CC) -x c -E -Wall -Werror /dev/null >/dev/null 2>&1 && echo 1)
   # Export CC_IS_GCC to save a shell invocation when recursing.
@@ -38,7 +44,7 @@ ifndef WARNING_CFLAGS
     disable_warning = $(shell $(CC) -x c -E -Werror -W$(1) /dev/null >/dev/null 2>&1 && echo -Wno-$(1))
 
     WARNING_CFLAGS = -Wall
-    ifeq ($(CC_NAME),clang)
+    ifdef CC_IS_CLANG
       # -Qunused-arguments : clang objects to arguments that it doesn't understand
       #    and fixing this would require rearchitecture
       WARNING_CFLAGS += -Qunused-arguments
@@ -62,10 +68,10 @@ ifndef WARNING_CFLAGS
         NSS_ENABLE_WERROR = 0
         $(warning OS_TARGET is Android, disabling -Werror)
       else
-        ifeq ($(CC_NAME),clang)
+        ifdef CC_IS_CLANG
           # Clang reports its version as an older gcc, but it's OK
           NSS_ENABLE_WERROR = 1
-        else ifeq ($(CC_NAME),gcc)
+        else ifdef CC_IS_GCC
           ifneq (,$(filter 4.8 4.9,$(word 1,$(GCC_VERSION)).$(word 2,$(GCC_VERSION))))
             NSS_ENABLE_WERROR = 1
           endif
diff --git a/lib/freebl/Makefile b/lib/freebl/Makefile
--- lib/freebl/Makefile
+++ lib/freebl/Makefile
@@ -484,7 +484,7 @@ endif
 
 # poly1305-donna-x64-sse2-incremental-source.c requires __int128 support
 # in GCC 4.6.0.
-ifeq ($(CC_NAME),clang)
+ifdef CC_IS_CLANG
     HAVE_INT128_SUPPORT = 1
 else ifeq (1,$(CC_IS_GCC))
     ifneq (,$(filter 4.6 4.7 4.8 4.9,$(word 1,$(GCC_VERSION)).$(word 2,$(GCC_VERSION))))
@@ -697,8 +697,8 @@ ifdef INTEL_GCM
 # symbolic names to registers, for example,
 #     .set  Htbl, %rdi
 # So we can't use Clang's integrated assembler with intel-gcm.s.
-ifneq (,$(findstring clang,$(shell $(AS) --version)))
-$(OBJDIR)/$(PROG_PREFIX)intel-gcm$(OBJ_SUFFIX): ASFLAGS += -no-integrated-as
+ifdef CC_IS_CLANG
+$(OBJDIR)/$(PROG_PREFIX)intel-gcm$(OBJ_SUFFIX): CFLAGS += -no-integrated-as
 endif
 endif