autotools: Fix static linking when openssl is enabled in windows

This adds Requires.private field in pkgconfig file. Using that field,
pkgconfig pulls all the private cflags or libs while static linking.
OpenSSL static libraries require some windows system libraies. Otherwise
static liking fails with libarchive.

build/pkgconfig/libarchive.pc.in

@@ -10,3 +10,4 @@ Cflags: -I${includedir}
 Cflags.private: -DLIBARCHIVE_STATIC
 Libs: -L${libdir} -larchive
 Libs.private: @LIBS@
+Requires.private: @LIBSREQUIRED@

configure.ac

@@ -378,6 +378,7 @@ if test "x$with_iconv" != "xno"; then
     AC_CHECK_HEADERS([localcharset.h])
     am_save_LIBS="$LIBS"
     LIBS="${LIBS} ${LIBICONV}"
+    LIBSREQUIRED="$LIBSREQUIRED${LIBSREQUIRED:+ }iconv"
     AC_CHECK_FUNCS([locale_charset])
     LIBS="${am_save_LIBS}"
     if test "x$ac_cv_func_locale_charset" != "xyes"; then
@@ -1209,6 +1210,7 @@ fi
 if test "x$with_openssl" != "xno"; then
     AC_CHECK_HEADERS([openssl/evp.h])
     saved_LIBS=$LIBS
+    LIBSREQUIRED="$LIBSREQUIRED${LIBSREQUIRED:+ }libssl libcrypto"
     AC_CHECK_LIB(crypto,OPENSSL_config)
     CRYPTO_CHECK(MD5, OPENSSL, md5)
     CRYPTO_CHECK(RMD160, OPENSSL, rmd160)
@@ -1219,6 +1221,8 @@ if test "x$with_openssl" != "xno"; then
     AC_CHECK_FUNCS([PKCS5_PBKDF2_HMAC_SHA1])
 fi
 
+AC_SUBST(LIBSREQUIRED)
+
 # Probe libmd AFTER OpenSSL/libcrypto.
 # The two are incompatible and OpenSSL is more complete.
 AC_CHECK_HEADERS([md5.h ripemd.h sha.h sha256.h sha512.h])