Add cross compilation support (#85)

Add support for cross-compiling curl-impersonate.

Cross compiling can now be done using the '--host' flag to the configure
script. This will make sure that all sub-components are cross-compiled.
In addition, compiling for a different system requires explicitly
specifying multiple paths used by curl (e.g. for certificates). These
options were added to the configure script as well.

The build and test CI workflow will now attempt to cross-compile
curl-impersonate to ARM64 (aarch64), and upload this binary to the
GitHub release page.

Add the 'configure' script generated by autoconf and its dependencies to
the repository to save the users from having to run 'autoconf' manually.

firefox/patches/curl-impersonate.patch

@@ -1246,10 +1246,10 @@ index 8ac15d407..68d01b219 100644
  Libs.private: @LIBCURL_LIBS@
  Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@
 diff --git a/m4/curl-nss.m4 b/m4/curl-nss.m4
-index 397ba71b1..d2a8fc1f2 100644
+index 397ba71b1..e7fe93925 100644
 --- a/m4/curl-nss.m4
 +++ b/m4/curl-nss.m4
-@@ -74,7 +74,107 @@ if test "x$OPT_NSS" != xno; then
+@@ -74,7 +74,123 @@ if test "x$OPT_NSS" != xno; then
        # Without pkg-config, we'll kludge in some defaults
        AC_MSG_WARN([Using hard-wired libraries and compilation flags for NSS.])
        addld="-L$OPT_NSS/lib"
@@ -1327,38 +1327,54 @@ index 397ba71b1..d2a8fc1f2 100644
 +      # at runtime using dlopen. If it's not in a path findable by dlopen
 +      # we have to add that path explicitly using -rpath so it may find it.
 +      # On Ubuntu and Mac M1 it is in a non-standard location.
-+      AC_MSG_CHECKING([if libnssckbi is in a non-standard location])
-+      case $host_os in
-+        linux*)
-+          search_paths="/usr/lib/$host /usr/lib/$host/nss"
-+          search_paths="$search_paths /usr/lib/$host_cpu-$host_os"
-+          search_paths="$search_paths /usr/lib/$host_cpu-$host_os/nss"
-+          search_ext="so"
-+          ;;
-+        darwin*)
-+          search_paths="/opt/homebrew/lib"
-+          search_ext="dylib"
-+          ;;
-+      esac
++      AC_ARG_WITH(libnssckbi,
++        [AS_HELP_STRING([--with-libnssckbi=DIRECTORY],
++                       [Path where libnssckbi can be found when using NSS])],
++        [AS_IF(
++          [test x"$withval" = xyes],
++          [nssckbi_path="check"],
++          [nssckbi_path="$withval"])],
++        [nssckbi_path="check"])
 +
-+      found="no"
-+      for path in $search_paths; do
-+        if test -f "$path/libnssckbi.$search_ext"; then
-+          AC_MSG_RESULT([$path])
-+          addld="$addld -Wl,-rpath,$path"
-+          found="yes"
-+          break
-+        fi
-+      done
++      AS_IF(
++        [test "x$nssckbi_path" = xno],
++        [],
++        [test "x$nssckbi_path" != xcheck],
++        [addld="$addld -Wl,-rpath,$nssckbi_path"],
++        [
++          AC_MSG_CHECKING([if libnssckbi is in a non-standard location])
++          case $host_os in
++            linux*)
++              search_paths="/usr/lib/$host /usr/lib/$host/nss"
++              search_paths="$search_paths /usr/lib/$host_cpu-$host_os"
++              search_paths="$search_paths /usr/lib/$host_cpu-$host_os/nss"
++              search_ext="so"
++              ;;
++            darwin*)
++              search_paths="/opt/homebrew/lib"
++              search_ext="dylib"
++              ;;
++          esac
 +
-+      if test "$found" = "no"; then
-+        AC_MSG_RESULT([no])
-+      fi
++          found="no"
++          for path in $search_paths; do
++            if test -f "$path/libnssckbi.$search_ext"; then
++              AC_MSG_RESULT([$path])
++              addld="$addld -Wl,-rpath,$path"
++              found="yes"
++              break
++            fi
++          done
++
++          if test "$found" = "no"; then
++            AC_MSG_RESULT([no])
++          fi
++        ])
 +
        addcflags="-I$OPT_NSS/include"
        version="unknown"
        nssprefix=$OPT_NSS
-@@ -91,7 +191,7 @@ if test "x$OPT_NSS" != xno; then
+@@ -91,7 +207,7 @@ if test "x$OPT_NSS" != xno; then
      fi
  
      dnl The function SSL_VersionRangeSet() is needed to enable TLS > 1.0
@@ -1367,7 +1383,7 @@ index 397ba71b1..d2a8fc1f2 100644
       [
       AC_DEFINE(USE_NSS, 1, [if NSS is enabled])
       AC_SUBST(USE_NSS, [1])
-@@ -101,9 +201,7 @@ if test "x$OPT_NSS" != xno; then
+@@ -101,9 +217,7 @@ if test "x$OPT_NSS" != xno; then
       test nss != "$DEFAULT_SSL_BACKEND" || VALID_DEFAULT_SSL_BACKEND=yes
       ],
       [