configure: Don't set user variables

CFLAGS is a user variable and shouldn't be set in either
configure.ac or Makefile.am because that makes it impossible to
override compiler flags at configure or make time.

AC_SUBST the default warning flags into AM_CFLAGS instead, which
is emitted before CFLAGS on the compiler command line.

Do the same thing with a mingw-specific flag as well.

Change-Id: I6ac3a3c8b7e0abf6f0e60ea239b549d68ea6d370
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3870
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

common.mk

@@ -1,6 +1,9 @@
 
 # common flags used in openocd build
-AM_CPPFLAGS = -I$(top_srcdir)/src \
+AM_CFLAGS = $(GCC_WARNINGS)
+
+AM_CPPFLAGS = $(HOST_CPPFLAGS)\
+			  -I$(top_srcdir)/src \
 			  -I$(top_builddir)/src \
 			  -I$(top_srcdir)/src/helper \
 			  -DPKGDATADIR=\"$(pkgdatadir)\" \

configure.ac

@@ -471,7 +471,7 @@ AS_CASE([$host],
       AC_MSG_ERROR([buspirate currently not supported by MinGW32 hosts])
     ])
 
-    CFLAGS="$CFLAGS -D__USE_MINGW_ANSI_STDIO"
+    AC_SUBST([HOST_CPPFLAGS], [-D__USE_MINGW_ANSI_STDIO])
 
     AC_DEFINE([IS_MINGW], [1], [1 if building for MinGW.])
     AC_DEFINE([IS_WIN32], [1], [1 if building for Win32.])
@@ -785,7 +785,7 @@ AS_IF([test "x${gcc_werror}" = "xyes"], [
 
 # overide default gcc cflags
 AS_IF([test "x$gcc_warnings" = "xyes"], [
-  CFLAGS="$CFLAGS $GCC_WARNINGS"
+  AC_SUBST([GCC_WARNINGS], [$GCC_WARNINGS])
 ])
 
 AC_CONFIG_FILES([