compiler wrapper: ensure variables are declared (#2744)

Variables like `mode` where not initialized, so that ultimately `$mode` referenced to the environment variable instead of the local variable.

Fix this class of issues for a couple local variables.

Author: sibcse

cc.sh

@@ -295,6 +295,10 @@ fi
 # Note. SPACK_ALWAYS_XFLAGS are applied for all compiler invocations,
 # including version checks (SPACK_XFLAGS variants are not applied
 # for version checks).
+mode=""
+vdep=""
+lang_flags=""
+debug_flags=""
 command="${0##*/}"
 comp="CC"
 vcheck_flags=""