Force generation of substitute .h file when C++ support is enabled.
* m4/ansi-c++.m4 (gl_ANSI_CXX): New macro.
* modules/ansi-c++-opt (configure.ac): Just require gl_ANSI_CXX.
* m4/fnmatch_h.m4 (gl_FNMATCH_H): If C++ support is enabled, set
FNMATCH_H to non-empty.
* m4/iconv_h.m4 (gl_ICONV_H_DEFAULTS): If C++ support is enabled, set
ICONV_H to non-empty.
* m4/monetary_h.m4 (gl_MONETARY_H_BODY): If C++ support is enabled, set
MONETARY_H to non-empty.
* m4/utime_h.m4 (gl_UTIME_H): If C++ support is enabled, set UTIME_H to
non-empty.