build: when building libsecp256k1, patch Makefile.am before autogen.sh

apparently this could have caused issues on MacOS

based on Electron-Cash/Electron-Cash@69f6cd0aa0552cb12bb2948da8f0d3f7ae32ffc8

contrib/build-linux/appimage/build.sh

@@ -86,8 +86,8 @@ info "building libsecp256k1."
     git reset --hard "$LIBSECP_VERSION"
     git clean -f -x -q
     export SOURCE_DATE_EPOCH=1530212462
-    ./autogen.sh
     echo "LDFLAGS = -no-undefined" >> Makefile.am
+    ./autogen.sh
     ./configure \
       --prefix="$APPDIR/usr" \
       --enable-module-recovery \

contrib/build-wine/build-secp256k1.sh

@@ -14,8 +14,8 @@ info "building libsecp256k1..."
 build_dll() {
     #sudo apt-get install -y mingw-w64
     export SOURCE_DATE_EPOCH=1530212462
-    ./autogen.sh
     echo "LDFLAGS = -no-undefined" >> Makefile.am
+    ./autogen.sh
     LDFLAGS="-Wl,--no-insert-timestamp" ./configure \
         --host=$1 \
         --enable-module-recovery \