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
 

depends/patches/boost/unused_var_in_process.patch

@@ -0,0 +1,22 @@
+commit dbd95cdaefdea95307d004f019a1c394cf9389f0
+Author: fanquake <fanquake@gmail.com>
+Date:   Mon Aug 17 20:15:17 2020 +0800
+
+    Remove unused variable in Boost Process
+
+    This causes issues with our linters / CI.
+
+    Can be removed once depends Boost is 1.71.0 or later.
+
+diff --git a/boost/process/detail/posix/wait_group.hpp b/boost/process/detail/posix/wait_group.hpp
+index 9dc249803..2502d9772 100644
+--- a/boost/process/detail/posix/wait_group.hpp
++++ b/boost/process/detail/posix/wait_group.hpp
+@@ -137,7 +137,6 @@ inline bool wait_until(
+
+     do
+     {
+-        int ret_sig = 0;
+         int status;
+         if ((::waitpid(timeout_pid, &status, WNOHANG) != 0)
+             && (WIFEXITED(status) || WIFSIGNALED(status)))