build-sys: Pass CFLAGS to $(DTRACE)

Fedora is using https://fedoraproject.org/wiki/Changes/Annobin
to try to ensure that all objects are built with hardening flags.
Pass down `CFLAGS` to ensure the SystemTap objects use them.
9 jobs for <span class="ref-name">backports-glib-2-58</span> in 29 minutes and 5 seconds (queued for 5 minutes and 57 seconds)
Status Job ID Name Coverage
  Build
passed #130296
cross-android_api21_arm64

00:01:29

passed #130297
cross-android_api28_arm64

00:01:32

passed #130298
cross-mingw64

00:02:18

passed #130295
fedora-autotools-x86_64

00:05:38

passed #130294
fedora-x86_64

00:05:16

manual #130301
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #130299
win32
msys2-mingw32

00:10:32

passed #130300
win32
vs2017-x64

00:07:36

 
  Coverage
passed #130302
coverage

00:02:07

74.5%