icu: support OpenWrt trunk host-build.mk issue

icu: support OpenWrt trunk host-build.mk issue

Maintainer: me
Compile tested: ar71xx, mips_34kc_gcc-5.3.0_musl, OpenWrt trunk 50104
                ar71xx, mips_24kc_gcc-5.4.0_musl, LEDE trunk r3503-a112435
Run tested: NONE

Description:
openwrt buildbot report build fail

```
./runConfigureICU CC="gcc" CFLAGS="-O2 -I/home/builder/trunk/openwrt/staging_dir/host/include -I/home/builder/trunk/openwrt/staging_dir/host/usr/include -I/home/builder/trunk/openwrt/staging_dir/target-i386_pentium4_glibc-2.22/host/include" CPPFLAGS="-I/home/builder/trunk/openwrt/staging_dir/host/include -I/home/builder/trunk/openwrt/staging_dir/host/usr/include -I/home/builder/trunk/openwrt/staging_dir/target-i386_pentium4_glibc-2.22/host/include" LDFLAGS="-L/home/builder/trunk/openwrt/staging_dir/host/lib -L/home/builder/trunk/openwrt/staging_dir/host/usr/lib -L/home/builder/trunk/openwrt/staging_dir/target-i386_pentium4_glibc-2.22/host/lib" SHELL="/usr/bin/env bash" Linux/gcc CC="gcc" CXX="g++" --disable-debug --enable-release --enable-shared --enable-static --enable-draft --enable-renaming --disable-tracing --disable-extras --enable-dyload --prefix=/home/builder/trunk/openwrt/staging_dir/target-i386_pentium4_glibc-2.22/host ; fi )
runConfigureICU: unrecognized platform "CC=gcc" (use --help for help)
```

"host-build.mk" is differs between OpenWrt and LEDE.

https://github.com/openwrt/packages/pull/3993

https://github.com/lede-project/source/commit/83b6bfc2357458f219872f97ed9c4894106131a1

Signed-off-by: Hirokazu MORIKAWA <morikw2@gmail.com>