elements/bootstrap/gcc.bst: don't set CFLAGS/CXXFLAGS/LDFLAGS to empty

- gcc's buildsystem treats empty and unset variables differently
- this is the only final stage component that does that
- previously they were unset and defaulted to "-g -O2" for CFLAGS/CXXFLAGS,
  but now they are empty so we lose on the optimization and debug info.

Fixes #784
11 jobs for abderrahim/gcc-cflags in 875 minutes and 52 seconds (queued for 15 seconds)
latest
Status Job ID Name Coverage
  Flatpak
passed #229873319
aarch64
app_aarch64

13:35:26

passed #229888859
armhf
app_arm

05:17:22

passed #229877215
x86_64 cache_i686
app_i686

04:53:43

passed #229915087
x86_64 cache_x86_64
app_x86_64

05:10:03

failed #229873324
armhf
app_arm

00:11:47

failed #229873309
cache_i686 x86_64
app_i686

00:00:01

failed #229873306
cache_x86_64 x86_64
app_x86_64

00:19:24

 
  Vm
manual #229873328
aarch64 allowed to fail manual
minimal_systemd_vm_aarch64
passed #229873329
armhf
minimal_systemd_vm_arm

00:03:28

passed #229873327
cache_i686 x86_64
minimal_systemd_vm_i686

00:02:26

passed #229873325
cache_x86_64 x86_64
minimal_systemd_vm_x86_64

00:02:19