Restructure *CFLAGS/*CXXFLAGS configuration.

Convert CFLAGS/CXXFLAGS to be concatenations:

  CFLAGS := CONFIGURE_CFLAGS SPECIFIED_CFLAGS EXTRA_CFLAGS
  CXXFLAGS := CONFIGURE_CXXFLAGS SPECIFIED_CXXFLAGS EXTRA_CXXFLAGS

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

This resolves #504.

INSTALL

@@ -307,17 +307,18 @@ 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.)
+CXXFLAGS="?"
+    Pass these flags to the C/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.
+EXTRA_CXXFLAGS="?"
+    Append these flags to CFLAGS/CXXFLAGS, without passing them to the
+    compiler(s) 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