configure: Append extra cflags and ldflags

In order to allow overriding flags that are set by configure, we have to
append them instead of prepending as it is done so far.

v2: Clarify documentation.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -380,9 +380,9 @@ for opt do
   ;;
   --install=*) install="$optarg"
   ;;
-  --extra-cflags=*) CFLAGS="$optarg"
+  --extra-cflags=*) EXTRA_CFLAGS="$optarg"
   ;;
-  --extra-ldflags=*) LDFLAGS="$optarg"
+  --extra-ldflags=*) EXTRA_LDFLAGS="$optarg"
   ;;
   --cpu=*) cpu="$optarg"
   ;;
@@ -601,8 +601,8 @@ echo "  --source-path=PATH       path of source code [$source_path]"
 echo "  --cross-prefix=PREFIX    use PREFIX for compile tools [$cross_prefix]"
 echo "  --cc=CC                  use C compiler CC [$cc]"
 echo "  --host-cc=CC             use C compiler CC [$host_cc] for dyngen etc."
-echo "  --extra-cflags=CFLAGS    add C compiler flags CFLAGS"
-echo "  --extra-ldflags=LDFLAGS  add linker flags LDFLAGS"
+echo "  --extra-cflags=CFLAGS    append extra C compiler flags CFLAGS"
+echo "  --extra-ldflags=LDFLAGS  append extra linker flags LDFLAGS"
 echo "  --make=MAKE              use specified make [$make]"
 echo "  --install=INSTALL        use specified install [$install]"
 echo "  --static                 enable static build [$static]"
@@ -1490,8 +1490,8 @@ echo "OS_CFLAGS=$OS_CFLAGS" >> $config_mak
 echo "OS_LDFLAGS=$OS_LDFLAGS" >> $config_mak
 echo "ARCH_CFLAGS=$ARCH_CFLAGS" >> $config_mak
 echo "ARCH_LDFLAGS=$ARCH_LDFLAGS" >> $config_mak
-echo "CFLAGS=$CFLAGS" >> $config_mak
-echo "LDFLAGS=$LDFLAGS" >> $config_mak
+echo "CFLAGS=$CFLAGS $EXTRA_CFLAGS" >> $config_mak
+echo "LDFLAGS=$LDFLAGS $EXTRA_LDFLAGS" >> $config_mak
 echo "EXESUF=$EXESUF" >> $config_mak
 echo "PTHREADLIBS=$PTHREADLIBS" >> $config_mak
 echo "CLOCKLIBS=$CLOCKLIBS" >> $config_mak