configure: ignore preexisting QEMU_*FLAGS envvars

User flags should be passed via CFLAGS/CXXFLAGS/LDFLAGS,
or --extra-cflags/extra-cxxflags/--extra-ldflags on the
command line.

QEMU_CFLAGS, QEMU_CXXFLAGS and QEMU_LDFLAGS are reserved
for flags detected by configure, so do not add to them
and clear them at the beginning of the script.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -158,7 +158,7 @@ update_cxxflags() {
     # Set QEMU_CXXFLAGS from QEMU_CFLAGS by filtering out those
     # options which some versions of GCC's C++ compiler complain about
     # because they only make sense for C programs.
-    QEMU_CXXFLAGS="$QEMU_CXXFLAGS -D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS"
+    QEMU_CXXFLAGS="-D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS"
     CONFIGURE_CXXFLAGS=$(echo "$CONFIGURE_CFLAGS" | sed s/-std=gnu11/-std=gnu++11/)
     for arg in $QEMU_CFLAGS; do
         case $arg in
@@ -465,11 +465,13 @@ sdl2_config="${SDL2_CONFIG-${cross_prefix}sdl2-config}"
 # left shift of signed integers is well defined and has the expected
 # 2s-complement style results. (Both clang and gcc agree that it
 # provides these semantics.)
-QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS"
+QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv"
 QEMU_CFLAGS="-Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS"
 QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS"
 QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE $QEMU_CFLAGS"
 
+QEMU_LDFLAGS=
+
 # Flags that are needed during configure but later taken care of by Meson
 CONFIGURE_CFLAGS="-std=gnu11 -Wall"
 CONFIGURE_LDFLAGS=