preprocessor: Support C2X #elifdef, #elifndef

C2X adds #elifdef and #elifndef preprocessor directives; these have
also been proposed for C++.  Implement these directives in libcpp
accordingly.

In this implementation, #elifdef and #elifndef are treated as
non-directives for any language version other than c2x and gnu2x (if
the feature is accepted for C++, it can trivially be enabled for
relevant C++ versions).  In strict conformance modes for prior
language versions, this is required, as illustrated by the
c11-elifdef-1.c test added.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

libcpp/
	* include/cpplib.h (struct cpp_options): Add elifdef.
	* init.c (struct lang_flags): Add elifdef.
	(lang_defaults): Update to include elifdef initializers.
	(cpp_set_lang): Set elifdef for pfile based on language.
	* directives.c (STDC2X, ELIFDEF): New macros.
	(EXTENSION): Increase value to 3.
	(DIRECTIVE_TABLE): Add #elifdef and #elifndef.
	(_cpp_handle_directive): Do not treat ELIFDEF directives as
	directives for language versions without the #elifdef feature.
	(do_elif): Handle #elifdef and #elifndef.
	(do_elifdef, do_elifndef): New functions.

gcc/testsuite/
	* gcc.dg/cpp/c11-elifdef-1.c, gcc.dg/cpp/c2x-elifdef-1.c,
	gcc.dg/cpp/c2x-elifdef-2.c: New tests.

3 files changed, 136 insertions, 0 deletions

diff --git a/gcc/testsuite/gcc.dg/cpp/c11-elifdef-1.c b/gcc/testsuite/gcc.dg/cpp/c11-elifdef-1.c
new file mode 100644
index 0000000..2d5809a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/c11-elifdef-1.c
@@ -0,0 +1,16 @@
+/* Test #elifdef and #elifndef not in C11.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#define A
+#undef B
+
+#if 0
+#elifdef A
+#error "#elifdef A applied"
+#endif
+
+#if 0
+#elifndef B
+#error "#elifndef B applied"
+#endif
diff --git a/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-1.c b/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-1.c
new file mode 100644
index 0000000..b23e311
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-1.c
@@ -0,0 +1,57 @@
+/* Test #elifdef and #elifndef in C2x.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+#define A
+#undef B
+
+#if 0
+#elifdef A
+#define M1 1
+#endif
+
+#if M1 != 1
+#error "#elifdef A did not apply"
+#endif
+
+#if 0
+#elifdef B
+#error "#elifdef B applied"
+#endif
+
+#if 0
+#elifndef A
+#error "#elifndef A applied"
+#endif
+
+#if 0
+#elifndef B
+#define M2 2
+#endif
+
+#if M2 != 2
+#error "#elifndef B did not apply"
+#endif
+
+#if 0
+#elifdef A
+#else
+#error "#elifdef A did not apply"
+#endif
+
+#if 0
+#elifndef B
+#else
+#error "#elifndef B did not apply"
+#endif
+
+/* As with #elif, the syntax of the new directives is relaxed after a
+   non-skipped group.  */
+
+#if 1
+#elifdef x * y
+#endif
+
+#if 1
+#elifndef !
+#endif
diff --git a/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-2.c b/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-2.c
new file mode 100644
index 0000000..9132832
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cpp/c2x-elifdef-2.c
@@ -0,0 +1,63 @@
+/* Test #elifdef and #elifndef in C2x: erroneous usages.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c2x -pedantic-errors" } */
+
+#define A
+#undef B
+
+#elifdef A /* { dg-error "#elifdef without #if" } */
+#elifdef B /* { dg-error "#elifdef without #if" } */
+#elifndef A /* { dg-error "#elifndef without #if" } */
+#elifndef B /* { dg-error "#elifndef without #if" } */
+
+#if 1 /* { dg-error "-:began here" } */
+#else
+#elifdef A /* { dg-error "#elifdef after #else" } */
+#endif
+
+#if 1 /* { dg-error "-:began here" } */
+#else
+#elifdef B /* { dg-error "#elifdef after #else" } */
+#endif
+
+#if 1 /* { dg-error "-:began here" } */
+#else
+#elifndef A /* { dg-error "#elifndef after #else" } */
+#endif
+
+#if 1 /* { dg-error "-:began here" } */
+#else
+#elifndef B /* { dg-error "#elifndef after #else" } */
+#endif
+
+#if 0
+#elifdef A = /* { dg-error "extra tokens at end of #elifdef directive" } */
+#endif
+
+#if 0
+#elifdef B = /* { dg-error "extra tokens at end of #elifdef directive" } */
+#endif
+
+#if 0
+#elifndef A = /* { dg-error "extra tokens at end of #elifndef directive" } */
+#endif
+
+#if 0
+#elifndef B = /* { dg-error "extra tokens at end of #elifndef directive" } */
+#endif
+
+#if 0
+#elifdef /* { dg-error "no macro name given in #elifdef directive" } */
+#endif
+
+#if 0
+#elifndef /* { dg-error "no macro name given in #elifndef directive" } */
+#endif
+
+#if 0
+#elifdef , /* { dg-error "macro names must be identifiers" } */
+#endif
+
+#if 0
+#elifndef , /* { dg-error "macro names must be identifiers" } */
+#endif