Makefile builds: do not inherit SRC from environment in big-int

In just `src/big-int` we were including `common` before defining `SRC`,
which meant a possible environment-defined value of `SRC` was being used
to construct some rules. The possible reason for having the `include`
earlier in just that file was that we were lacking a rule for `.cc`
files, the absence of which caused `make` to fail when sourcing `common`
after setting `SRC`.

Both fixes together now ensure that even an environment with `SRC` set
won't cause builds to fail in strange ways.

Fixes: #4834

Author: tautschnig

src/big-int/makefile

@@ -1,7 +1,8 @@
+SRC = bigint-func.cc bigint.cc
+
 include ../config.inc
 include ../common
 
-SRC = bigint-func.cc bigint.cc
 CLEANFILES = big-int$(LIBEXT)
 
 all: big-int$(LIBEXT)

src/common

@@ -270,6 +270,7 @@ sources: Makefile
 # endif
 
 D_FILES1 = $(SRC:.c=$(DEPEXT))
-D_FILES = $(D_FILES1:.cpp=$(DEPEXT))
+D_FILES2 = $(D_FILES1:.cc=$(DEPEXT))
+D_FILES = $(D_FILES2:.cpp=$(DEPEXT))
 
 -include $(D_FILES)