build: create a standalone build script for libsecp256k1

heavily based on Electron-Cash/Electron-Cash@eda015908e

contrib/make_libsecp256k1.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+
+LIBSECP_VERSION="b408c6a8b287003d1ade5709e6f7bc3c7f1d5be7"
+
+set -e
+
+here=$(dirname $(realpath "$0" 2> /dev/null || grealpath "$0"))
+CONTRIB="$here"
+PROJECT_ROOT="$CONTRIB/.."
+
+. "$here"/build_tools_util.sh || (echo "Could not source build_tools_util.sh" && exit 1)
+
+pkgname="secp256k1"
+info "Building $pkgname..."
+
+(
+    cd $CONTRIB
+    if [ ! -d secp256k1 ]; then
+        git clone https://github.com/bitcoin-core/secp256k1.git
+    fi
+    cd secp256k1
+    git reset --hard
+    git clean -f -x -q
+    git checkout $LIBSECP_VERSION
+
+    if ! [ -x configure ] ; then
+        echo "libsecp256k1_la_LDFLAGS = -no-undefined" >> Makefile.am
+        echo "LDFLAGS = -no-undefined" >> Makefile.am
+        ./autogen.sh || fail "Could not run autogen for $pkgname. Please make sure you have automake and libtool installed, and try again."
+    fi
+    if ! [ -r config.status ] ; then
+        ./configure \
+            $AUTOCONF_FLAGS \
+            --prefix="$here/$pkgname/dist" \
+            --enable-module-recovery \
+            --enable-experimental \
+            --enable-module-ecdh \
+            --disable-jni \
+            --disable-tests \
+            --disable-static \
+            --enable-shared || fail "Could not configure $pkgname. Please make sure you have a C compiler installed and try again."
+    fi
+    make -j4 || fail "Could not build $pkgname"
+    make install || fail "Could not install $pkgname"
+    . "$here/$pkgname/dist/lib/libsecp256k1.la"
+    host_strip "$here/$pkgname/dist/lib/$dlname"
+    cp -fpv "$here/$pkgname/dist/lib/$dlname" "$PROJECT_ROOT/electrum" || fail "Could not copy the $pkgname binary to its destination"
+    info "$dlname has been placed in the inner 'electrum' folder."
+)