[PATCH] Fix generation of C code for C++ compilers.

[Vincent Lefevre]

AC_CHECK_LIB fails with a C++ compiler (e.g., g++) because
  #ifdef __cplusplus
  extern "C"
  #endif
is missing in the generated C code. These 3 lines were removed in
commit 326c9a547423d25c621bc5c0ef76edbf6eda8c92 with the comment
"(AC_LANG_CALL(C)): Remove #ifdef __cplusplus, this macro is no longer
used to generate C++ code."
But this is actually C code compiled with a C++ compiler, not C++ code.
---
 lib/autoconf/c.m4 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/autoconf/c.m4 b/lib/autoconf/c.m4
index 44443a39..3ae432bc 100644
--- a/lib/autoconf/c.m4
+++ b/lib/autoconf/c.m4
@@ -127,6 +127,9 @@ m4_if([$2], [main], ,
 [/* Override any GCC internal prototype to avoid an error.
    Use char because int might match the return type of a GCC
    builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
 char $2 ();])], [return $2 ();])])
 
 
-- 
2.33.0