tests: check that C and C++ objects can be linked together.

* tests/local.at (AT_SKIP_IF_CANNOT_LINK_C_AND_CXX): New.
* tests/headers.at (Several parsers): Use it.

tests/headers.at

@@ -240,9 +240,6 @@ AT_TEST([x7], [%define api.push-pull both])
 AT_TEST([x8], [%define api.pure %define api.push-pull both])
 #AT_TEST([x5], [%locations %language "c++" %glr-parser])
 
-AT_COMPILE_CXX([parser], [[x[1-8].o -DCC_IS_CXX=$CC_IS_CXX main.cc]])
-AT_CHECK([./parser], [0], [[expout]])
-
 # Check that the headers are self-contained, and protected against
 # multiple inclusions.  While at it, check they are sane for C++.
 for h in *.h *.hh
@@ -256,6 +253,12 @@ EOF
   AT_COMPILE_CXX([$h.o])
 done
 
+# Do this late, so that other checks have been performed.
+AT_SKIP_IF_CANNOT_LINK_C_AND_CXX
+
+AT_COMPILE_CXX([parser], [[x[1-8].o -DCC_IS_CXX=$CC_IS_CXX main.cc]])
+AT_CHECK([./parser], [0], [[expout]])
+
 m4_popdef([AT_TEST])
 
 AT_CLEANUP

tests/local.at

@@ -683,6 +683,44 @@ m4_define([AT_FULL_COMPILE],
 ])
 
 
+
+# AT_SKIP_IF_CANNOT_LINK_C_AND_CXX
+# --------------------------------
+# Check that we can link together C and C++ objects.
+m4_define([AT_SKIP_IF_CANNOT_LINK_C_AND_CXX],
+[AT_DATA([c-and-cxx.h],
+[[#ifdef __cplusplus
+extern "C"
+{
+#endif
+  int fortytwo (void);
+#ifdef __cplusplus
+}
+#endif
+]])
+AT_DATA([c-only.c],
+[[#include "c-and-cxx.h"
+int
+main (void)
+{
+  return fortytwo () == 42 ? 0 : 1;
+}
+]])
+AT_DATA([cxx-only.cc],
+[[#include "c-and-cxx.h"
+int fortytwo ()
+{
+  return 42;
+}
+]])
+AT_COMPILE([c-only.o], [c-only.c])
+AT_COMPILE_CXX([cxx-only.o], [cxx-only.cc])
+AT_CHECK([$CXX $CXXFLAGS $CPPFLAGS $LDFLAGS c-only.o cxx-only.o -o c-and-cxx ||
+          exit 77], [ignore], [ignore])
+AT_CHECK([./c-and-cxx])
+])
+
+
 ## ---------------------------- ##
 ## Running a generated parser.  ##
 ## ---------------------------- ##