Use exact compiler for dkms as used to build the kernel

Ubuntu kernel builds do not export .kernelvariables file, thus extend
CC detection to read compiler binary from the .config file

Gbp-Pq: Name exact-cc.patch

dkms.in

@@ -1006,6 +1006,13 @@ actual_build()
         unset CC
     fi
 
+    if [[ -e $kernel_source_dir/.config ]]; then
+        cc=$(sed -n 's|^CONFIG_CC_VERSION_TEXT="\([^ ]*\) .*"|\1|p' $kernel_source_dir/.config)
+        if command -v "$cc" >/dev/null; then
+            export CC="$cc"
+        fi
+    fi
+
     invoke_command "$clean" "Cleaning build area" background
     echo $"DKMS make.log for $module-$module_version for kernel $kernelver ($arch)" >> "$build_log"
     date >> "$build_log"