[Bug bootstrap/78251] config/gettext.m4 and config/iconv.m4 contaminate CPPFLAGS

[howarth.at.gcc at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78251

--- Comment #2 from Jack Howarth <howarth.at.gcc at gmail dot com> ---
It appears that config/iconv.m4 needs to be reworked for its tests to succeed.
Removing  INCICONV from CPPFLAGS on darwin causes the headers from /usr/include
to be accidentally used against the libs from /opt/local/lib...

configure:10761: /usr/bin/clang++ -arch x86_64 -std=gnu++98 -o conftest -g    
-Wl,-no_pie  conftest.cpp  -L/opt/local/lib -liconv >&5

fails because -I/opt/local/include was dropped. This should be available to
configure already from...

--with-libiconv-prefix[=DIR]

without resorting to CPPFLAGS to pass it around to the Makefile.