contrib: add instructions re cross-compiling libsecp to Linux x86

related: #6669

(note that instructions assume this commit as otherwise AUTOCONF_FLAGS is overwritten!)

based on https://stackoverflow.com/a/17748092

contrib/build_tools_util.sh

@@ -119,8 +119,6 @@ export SOURCE_DATE_EPOCH=1530212462
 export PYTHONHASHSEED=22
 # Set the build type, overridden by wine build
 export BUILD_TYPE="${BUILD_TYPE:-$(uname | tr '[:upper:]' '[:lower:]')}"
-# No additional autoconf flags by default
-export AUTOCONF_FLAGS=""
 # Add host / build flags if the triplets are set
 if [ -n "$GCC_TRIPLET_HOST" ] ; then
     export AUTOCONF_FLAGS="$AUTOCONF_FLAGS --host=$GCC_TRIPLET_HOST"

contrib/make_libsecp256k1.sh

@@ -9,6 +9,10 @@
 # $ GCC_TRIPLET_HOST="i686-w64-mingw32" ./contrib/make_libsecp256k1.sh
 # Or for a Windows x86_64 (64-bit) target, run:
 # $ GCC_TRIPLET_HOST="x86_64-w64-mingw32" ./contrib/make_libsecp256k1.sh
+#
+# To cross-compile to Linux x86:
+# sudo apt-get install gcc-multilib g++-multilib
+# $ AUTOCONF_FLAGS="--host=i686-linux-gnu CFLAGS=-m32 CXXFLAGS=-m32 LDFLAGS=-m32" ./contrib/make_libsecp256k1.sh
 
 LIBSECP_VERSION="dbd41db16a0e91b2566820898a3ab2d7dad4fe00"