restructure *CFLAGS configuration.

Convert CFLAGS to be a concatenation:

  CFLAGS := CONFIGURE_CFLAGS SPECIFIED_CFLAGS EXTRA_CFLAGS

This ordering makes it possible to override the flags set by the
configure script both during and after configuration, with CFLAGS and
EXTRA_CFLAGS, respectively.

This resolves jemalloc#619.

Author: jasone

INSTALL

@@ -306,17 +306,16 @@ The following environment variables (not a definitive list) impact configure's
 behavior:
 
 CFLAGS="?"
-    Pass these flags to the compiler.  You probably shouldn't define this unless
-    you know what you are doing.  (Use EXTRA_CFLAGS instead.)
+    Pass these flags to the C compiler.  Any flags set by the configure script
+    are prepended, which means explicitly set flags generally take precedence.
+    Take care when specifying flags such as -Werror, because configure tests may
+    be affected in undesirable ways.
 
 EXTRA_CFLAGS="?"
-    Append these flags to CFLAGS.  This makes it possible to add flags such as
-    -Werror, while allowing the configure script to determine what other flags
-    are appropriate for the specified configuration.
-
-    The configure script specifically checks whether an optimization flag (-O*)
-    is specified in EXTRA_CFLAGS, and refrains from specifying an optimization
-    level if it finds that one has already been specified.
+    Append these flags to CFLAGS, without passing them to the compiler during
+    configuration.  This makes it possible to add flags such as -Werror, while
+    allowing the configure script to determine what other flags are appropriate
+    for the specified configuration.
 
 CPPFLAGS="?"
     Pass these flags to the C preprocessor.  Note that CFLAGS is not passed to

Makefile.in

@@ -24,8 +24,10 @@ abs_objroot := @abs_objroot@
 
 # Build parameters.
 CPPFLAGS := @CPPFLAGS@ -I$(srcroot)include -I$(objroot)include
+CONFIGURE_CFLAGS := @CONFIGURE_CFLAGS@
+SPECIFIED_CFLAGS := @SPECIFIED_CFLAGS@
 EXTRA_CFLAGS := @EXTRA_CFLAGS@
-CFLAGS := @CFLAGS@ $(EXTRA_CFLAGS)
+CFLAGS := $(strip $(CONFIGURE_CFLAGS) $(SPECIFIED_CFLAGS) $(EXTRA_CFLAGS))
 LDFLAGS := @LDFLAGS@
 EXTRA_LDFLAGS := @EXTRA_LDFLAGS@
 LIBS := @LIBS@