While most systems provide a separate iconv library against which
applications can link, musl based systems do not provide such a library.
Instead, iconv functions are directly included in the C library. As our
current CMake module to locate the iconv library only checks whether a
library exists somewhere in the typical library directories, we will
never build libgit2 with libiconv support on such systems.

Extend the iconv module to also search whether libc provides iconv
functions, which we do by checking whether the `iconv_open` function
exists inside of libc. If this is the case, we will default to use the
libc provided one instead of trying to use a separate libiconv. While
this changes which iconv we use on systems where both libc and an
external libiconv exist, to the best of my knowledge common systems only
provide either one or the other.

Note that libiconv support in musl is held kind of basic. To quote musl
libc's page on functional differences from glibc [1]:

    The iconv implementation musl is very small and oriented towards
    being unobtrusive to static link. Its character set/encoding
    coverage is very strong for its size, but not comprehensive like
    glibc’s.

As we assume iconv to be a lot more capable than what musl provides,
some of our tests will fail if using iconv on musl-based platforms.

[1]: https://wiki.musl-libc.org/functional-differences-from-glibc.html

We do want to notify users compiling our source code early on if they
try to use C flags which aren't supported. Add a new macro `AddCFlag`,
which results in a fatal error in case the flag is not supported, and
use it for our fuzzing flags.

Libraries found by CMake modules are usually handled with their full
path. This makes linking against those libraries a lot more robust when
it comes to libraries in non-standard locations, as otherwise we might
mix up libraries from different locations when link directories are
given.

One excemption are libraries found by PKG_CHECK_MODULES. Instead of
returning libraries with their complete path, it will return the
variable names as well as a set of link directories. In case where
multiple sets of the same library are installed in different locations,
this can lead the compiler to link against the wrong libraries in the
end, when link directories of other dependencies are added.

To fix this shortcoming, we need to manually resolve library paths
returned by CMake against their respective library directories. This is
an easy task to do with `FIND_LIBRARY`.

Move the odd code that provides a hierarchical display for projects
within the IDEs to its own module.

Move the nanosecond detection in time structures to its own module.

Since OpenSSL isn't used any more on OS X, there is no dependency
on any MacPorts library under /opt/local and there is no danger of
conflicts between MacPorts and system iconv. For this reason the
system iconv can always be used now.