coq-par-compile: add user options for extra coqc/coqdep arguments

User options coq-compile-extra-coqc-arguments and
coq-compile-extra-coqdep-arguments are added as list of arguments to
invocation of, respetively, coqc and coqdep in addition to the
arguments computed, e.g., from _CoqProject.

This can be used to work around #724.

coq/coq-compile-common.el

@@ -404,6 +404,22 @@ the ``-vok'' second stage was implemented."
   :type 'number
   :safe 'numberp)
 
+(defcustom coq-compile-extra-coqdep-arguments nil
+  "Additional coqdep arguments for auto compilation as list of strings.
+These arguments are added to all coqdep invocations during
+automatic compilation in addition to the arguments computed
+automatically, for instance by parsing a _CoqProject file."
+  :type '(repeat string)
+  :safe (lambda (v) (cl-every #'stringp v)))
+
+(defcustom coq-compile-extra-coqc-arguments nil
+  "Additional coqc arguments for auto compilation as list of strings.
+These arguments are added to all coqc invocations during
+automatic compilation in addition to the arguments computed
+automatically, for instance by parsing a _CoqProject file."
+  :type '(repeat string)
+  :safe (lambda (v) (cl-every #'stringp v)))
+
 (defcustom coq-compile-command ""
   "External compilation command.  If empty ProofGeneral compiles itself.
 If unset (the empty string) ProofGeneral computes the dependencies of

coq/coq-par-compile.el

@@ -761,7 +761,8 @@ earlier than 8.19."
 Argument CLPATH must be `coq-load-path' from the buffer
 that triggered the compilation, in order to provide correct
 load-path options to coqdep."
-  (nconc (coq-par-coqdep-warning-arguments)
+  (nconc (copy-sequence coq-compile-extra-coqdep-arguments) ; copy for nconc
+         (coq-par-coqdep-warning-arguments)
          (coq-coqdep-prog-args clpath
                                (file-name-directory lib-src-file)
                                (coq--pre-v85))
@@ -772,7 +773,10 @@ load-path options to coqdep."
 Argument CLPATH must be `coq-load-path' from the buffer
 that triggered the compilation, in order to provide correct
 load-path options to coqdep."
-  (nconc (coq-coqc-prog-args clpath (file-name-directory lib-src-file) (coq--pre-v85))
+  (nconc (copy-sequence coq-compile-extra-coqc-arguments) ; copy for nconc
+         (coq-coqc-prog-args clpath
+                             (file-name-directory lib-src-file)
+                             (coq--pre-v85))
          (list lib-src-file)))
 
 (defun coq-par-analyse-coq-dep-exit (status output command)