build: use patch rather than sed in Boost package

The depends comment is actually incorrect, and this can be dropped once
we move to 1.71.0 or later.

depends/packages/boost.mk

@@ -3,6 +3,7 @@ $(package)_version=1_70_0
 $(package)_download_path=https://dl.bintray.com/boostorg/release/1.70.0/source/
 $(package)_file_name=boost_$($(package)_version).tar.bz2
 $(package)_sha256_hash=430ae8354789de4fd19ee52f3b1f739e1fba576f0aded0897c3c2bc00fb38778
+$(package)_patches=unused_var_in_process.patch
 
 define $(package)_set_vars
 $(package)_config_opts_release=variant=release
@@ -31,9 +32,8 @@ $(package)_cxxflags_linux=-fPIC
 $(package)_cxxflags_android=-fPIC
 endef
 
-# Fix unused variable in boost_process, can be removed after upgrading to 1.72
 define $(package)_preprocess_cmds
-  sed -i.old "s/int ret_sig = 0;//" boost/process/detail/posix/wait_group.hpp && \
+  patch -p1 < $($(package)_patch_dir)/unused_var_in_process.patch && \
   echo "using $($(package)_toolset_$(host_os)) : : $($(package)_cxx) : <cxxflags>\"$($(package)_cxxflags) $($(package)_cppflags)\" <linkflags>\"$($(package)_ldflags)\" <archiver>\"$($(package)_archiver_$(host_os))\" <striper>\"$(host_STRIP)\"  <ranlib>\"$(host_RANLIB)\" <rc>\"$(host_WINDRES)\" : ;" > user-config.jam
 endef