macros: pass CFLAGS/CXXFLAGS to the processes started by cargo

This is mainly targeted to the crates using `rust-cc`, where we want to
apply system-wide compiler flags when invoking the compiler. `cc`
supports CFLAGS and CXXFLAGS, but we never set those.
Defining that per project is not possible, as we have no way to inherit
any build context from a dependency package.

Possibly also affects: `bindgen`, `cxx`, any other crate that invokes
the C/C++ compiler and passes the CFLAGSS

data/macros.cargo

@@ -22,6 +22,11 @@ rustc = "%{__rustc}"\
 rustdoc = "%{__rustdoc}"\
 rustflags = %{__global_rustflags_toml}\
 \
+[env]\
+CFLAGS = "%{build_cflags}"\
+CXXFLAGS = "%{build_cxxflags}"\
+LDFLAGS = "%{build_ldflags}"\
+\
 [install]\
 root = "%{buildroot}%{_prefix}"\
 \