has_icu.exe requires debug ICU libraries on release build

[chr-thien]

I'm building the release variant of Boost 1.72 using MSVC 14.
I would like to link Regex to ICU.
ICU has standard library names.
ICU release build libraries are located in lib64 subdirectory of ICU_PATH parameter.
There are no debug build libraries of ICU.

b2.exe --reconfigure --prefix=<path> --layout=versioned --with-regex -j4 -sICU_PATH=<path> toolset=msvc-14.0 address-model=64 link=shared runtime-link=shared variant=release inlining=off optimization=off debug-symbols=on install

During release build ICU is not found because it requires debug libraries of ICU to be present.
has_icu.exe is always built as debug variant and therefore wants to link to icuucd, icuind.

The release build should use release libraries of ICU for all targets.

I know there is ICU_ICUUC_NAME for overriding. But when using standard library names the automatic detection should work reliably.

[jzmaddock]

Moved upstream to Boost.Build.