icu: pass in install program as native path, so Win32 make can invoke it

No idea why this wasn't a problem before.

Change-Id: Icbb94111061079069e2fc2102cfa4fe4bdd59bb9

external/icu/ExternalProject_icu.mk

@@ -23,6 +23,7 @@ $(call gb_ExternalProject_get_state_target,icu,build) :
 	$(call gb_ExternalProject_run,build,\
 		export LIB="$(ILIB)" \
 		&& CFLAGS="$(SOLARINC) $(gb_DEBUG_CFLAGS)" CPPFLAGS="$(SOLARINC)" CXXFLAGS="$(SOLARINC) $(gb_DEBUG_CFLAGS)" \
+			INSTALL=`cygpath -m /usr/bin/install` \
 			./runConfigureICU \
 			$(if $(MSVC_USE_DEBUG_RUNTIME),--enable-debug --disable-release) \
 			Cygwin/MSVC \