Fix compilation errors in test-math-c++.cc on FreeBSD 12.2/arm64

[Bruno Haible]

On FreeBSD 12.2/arm64, when compiling a testdir of the POSIX modules
with C++ tests included, I get compilation errors in test-math-c++.cc.
The error flood starts with

In file included from ../../gltests/test-math-c++.cc:22:
In file included from ../gllib/math.h:27:
In file included from /usr/include/c++/v1/math.h:301:
In file included from ../gllib/stdlib.h:46:
In file included from ../gllib/sys/wait.h:28:
In file included from /usr/include/sys/wait.h:156:
In file included from ../gllib/sys/types.h:39:
In file included from /usr/include/sys/types.h:376:
In file included from ../gllib/sys/select.h:88:
In file included from ../gllib/sys/time.h:39:
In file included from /usr/include/sys/time.h:607:
In file included from ../gllib/sys/select.h:117:
In file included from ../gllib/signal.h:67:
In file included from ../gllib/pthread.h:56:
In file included from ../gllib/stdlib.h:28:
In file included from /usr/include/c++/v1/stdlib.h:100:
../gllib/math.h:740:19: error: no member named 'acosf' in the global namespace; 
did you mean 'atof'?

This patch fixes it.


2020-12-02  Bruno Haible  <bruno@clisp.org>

        Fix compilation errors in test-math-c++.cc on FreeBSD 12.2/arm64.
        * lib/math.in.h (_GL_INCLUDING_MATH_H): New macro, to work around
        recursive self-include problem on FreeBSD 12.2 in C++ mode.

diff --git a/lib/math.in.h b/lib/math.in.h
index a020271..1d7b703 100644
--- a/lib/math.in.h
+++ b/lib/math.in.h
@@ -22,8 +22,23 @@
 #endif
 @PRAGMA_COLUMNS@
 
+#if defined _GL_INCLUDING_MATH_H
+/* Special invocation convention:
+   - On FreeBSD 12.2 we have a sequence of nested includes
+     <math.h> -> <stdlib.h> -> <sys/wait.h> -> <sys/types.h> -> <sys/select.h>
+       -> <signal.h> -> <pthread.h> -> <stdlib.h> -> <math.h>
+     In this situation, the functions are not yet declared, therefore we cannot
+     provide the C++ aliases.  */
+
+#@INCLUDE_NEXT_AS_FIRST_DIRECTIVE@ @NEXT_AS_FIRST_DIRECTIVE_MATH_H@
+
+#else
+/* Normal invocation convention.  */
+
 /* The include_next requires a split double-inclusion guard.  */
+#define _GL_INCLUDING_MATH_H
 #@INCLUDE_NEXT_AS_FIRST_DIRECTIVE@ @NEXT_AS_FIRST_DIRECTIVE_MATH_H@
+#undef _GL_INCLUDING_MATH_H
 
 #ifndef _@GUARD_PREFIX@_MATH_H
 #define _@GUARD_PREFIX@_MATH_H
@@ -2601,4 +2616,5 @@ _GL_WARN_REAL_FLOATING_DECL (signbit);
 _GL_INLINE_HEADER_END
 
 #endif /* _@GUARD_PREFIX@_MATH_H */
+#endif /* _GL_INCLUDING_MATH_H */
 #endif /* _@GUARD_PREFIX@_MATH_H */