Fix compilation errors in C++ mode on Solaris 10 and Solaris 11.
* m4/isfinite.m4 (gl_ISFINITE): Require AC_CANONICAL_HOST. On Solaris,
set REPLACE_ISFINITE to 1.
* m4/isinf.m4 (gl_ISINF): Require AC_CANONICAL_HOST. On Solaris, set
REPLACE_ISINF to 1.
* m4/signbit.m4 (gl_SIGNBIT): On Solaris, set REPLACE_SIGNBIT to 1.
* lib/pthread.in.h (pthread_create, pthread_once, pthread_key_create):
Use _GL_CXXALIAS_SYS_CAST instead of _GL_CXXALIAS_SYS.
* lib/threads.in.h (call_once): Likewise.
* lib/iconv.in.h (iconv): Likewise.