package/gpsd: fix workaround for gcc bug 68485

package/gpsd: fix workaround for gcc bug 68485

Workaround for gcc bug 68485 doesn't work anymore since bump to version
3.21 in commit 5b3e72139952e0c953c8649cca55571cec5f3836 because CXXFLAGS
is appended after CFLAGS and does not contain -O0:

/tmp/instance-7/output-1/host/bin/microblazeel-linux-gcc -o rtcm2_json.os -c --sysroot=/tmp/instance-7/output-1/host/microblazeel-buildroot-linux-uclibc/sysroot -pthread -Wall -Wcast-align -Wextra -Wimplicit-fallthrough -Wmissing-declarations -Wmissing-prototypes -Wno-missing-field-initializers -Wno-uninitialized -Wpointer-arith -Wreturn-type -Wstrict-prototypes -Wvla -O0 -Os -g0 -pthread -fPIC -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -I/tmp/instance-7/output-1/host/microblazeel-buildroot-linux-uclibc/sysroot/usr/include/dbus-1.0 -I/tmp/instance-7/output-1/host/microblazeel-buildroot-linux-uclibc/sysroot/usr/lib/dbus-1.0/include rtcm2_json.c
during RTL pass: reload
rtcm2_json.c: In function ‘json_rtcm2_read’:
rtcm2_json.c:267:1: internal compiler error: in gen_reg_rtx, at emit-rtl.c:1155
  267 | }
      | ^

It should be noted that GPSD_CXXFLAGS was added 5 years ago with commit
b4c050e9afed7a8dc302e6e1014cc4ffb508590d but was not actually used in
that commit or any later.

Fixes:
 - http://autobuild.buildroot.org/results/fab33f25b08fa218af91640fdbd8c0dcf1d07228

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>