Ensure Iconv is found when provided via CFLAGS/LDFLAGS

[Youw]

• by default find_file/find_library doesn't respect CFLAGS/LDFLAGS, and FindIconv fails to find Iconv;
• by explicitly trying to link against -liconv - we're checking if library is available in such way;
• additionally: if Iconv is detected as BUILT_IN, no need to explicitly depend on Iconv::Iconv;

Fixes: #429

[Youw]

Right now I don't have the environment to check this properly (except what is setup on CI).

@neheb can you check this as part of the openwrt build?

[neheb]

As soon as I get home.

[neheb]

nope:

-- The C compiler identification is GNU 12.1.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /home/mangix/devstuff/openwrt/staging_dir/toolchain-mips_24kc_gcc-12.1.0_musl/bin/mips-openwrt-linux-musl-gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- hidapi: v0.12.0
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Found PkgConfig: /home/mangix/devstuff/openwrt/staging_dir/host/bin/pkg-config (found version "1.8.0")
-- Checking for module 'libudev'
--   Found libudev, version 243
-- Checking for module 'libusb-1.0>=1.0.9'
--   Found libusb-1.0, version 1.0.24
-- Performing Test Iconv_IS_BUILT_IN
-- Performing Test Iconv_IS_BUILT_IN - Failed
-- Could NOT find Iconv (missing: Iconv_LIBRARY)
-- Could NOT find Iconv (missing: Iconv_LIBRARY)
CMake Error at libusb/CMakeLists.txt:41 (message):
  Iconv is not found, make sure your build environment is right


-- Configuring incomplete, errors occurred!
See also "/home/mangix/devstuff/openwrt/build_dir/target-mips_24kc_musl/hidapi-hidapi-0.12.0/CMakeFiles/CMakeOutput.log".
See also "/home/mangix/devstuff/openwrt/build_dir/target-mips_24kc_musl/hidapi-hidapi-0.12.0/CMakeFiles/CMakeError.log".

[Youw]

Updated. This time I was able to test it partially.

[neheb]

Seems to have done it. A lot of work as compared to

dependency('iconv')

though.

[Youw]

Not a lot of work, compared to uncompleted #426 :)

Besides, I'll ask CMake community, if this can be improved by CMake itself, even though openwrt CMake support is too incomplete as of yet.

---

Thanks for testing it.

[neheb]

I do agree CMake's Find_Iconv is not that great, Fixing it upstream would be good.

The OpenWrt comment is interesting. I don't really know what the "proper" way of handling cross compilation is like this. Although cmake.mk probably predates whatever "proper" is.

[Youw]

I don't really know what the "proper" way of handling cross compilation is like this.

Let me share with you a few links, as a general reference:

• The best way to cross-compile with CMake is to prepare a CMake toolchain file. Here is the official documentation: https://cmake.org/cmake/help/book/mastering-cmake/chapter/Cross%20Compiling%20With%20CMake.html
  So instead of specifying so many CMake arguments and environment varibales - a single additional CMake argument should be passed: -DCMAKE_TOOLCHAIN_FILE=<toolchain for cross-compilation>;
• a very good example of CMake toolchain file is the one provided by Android NDK. The one I used recently is located at: ndk/22.1.7171670/build/cmake/android.toolchain.cmake;
• some while ago I've been using Yocto builds for one of the development boards; Yocto has mature CMake support - that I remember, even though cannot really share a specific link to check out;

---

BTW:

the one provided by Android

I'm successfully using with HIDAPI (and not only) in several of my projects.