[Bug] iconv dependency needs a better configuration on some systems

[wenzeslaus]

Describe the bug
When compiling in some environments such conda (where iconv is installed through conda, not in the system) and at least some old FreeBSD, compilation fails.

To Reproduce
You need a system with special iconv or perhaps no iconv. I don't have any instructions to do that at this point.

Expected behavior
If the automatic detection fails, there should be way in ./configure to specify include and library paths for iconv as well as a name of the library itself. It should be also possible to disable inconv completely like other dependencies.

System description (please complete the following information):

• CentOS Linux release 7.7.1908 (Core) inside conda, perhaps FreeBSD
• master or 7.8 branch

Additional context
GRASS compiles and works fine with manually disabled iconv by setting #define HAVE_ICONV_H 0 in include/config.h (220) after ./configure.

Given the large number of dependencies, I think it would not be additional burden if the automatic search for iconv would be disabled and explicit settings would be required for more cases than now, but I don't know how this would play out with the iconv in libc.

According to @metzm and @glynnc the problem is that in the configure script, the header checks and the library checks are disconnected (see links above for details).

This is something to keep in mind for #348 (CMake).