build, mingw: set LIBS in configure.ac

Instead of doing it instead Makefile.am, this fixes libuv.pc on MinGW.

Previous version generates libuv.pc with LIBS = "-lpthread" instead of
"-lpthread -lws2_32 -lpsapi -liphlpapi -lshell32 -luserenv" on MinGW.

Previous version generates the following Makefile.am:

    am__append_4 = -lws2_32 -lpsapi -liphlpapi -lshell32 -luserenv
    ...
    LIBS = -lpthread  $(am__append_4)

and the following configure.ac:

    AS_IF([test "x$PKG_CONFIG" != "x"], [
        AC_CONFIG_FILES([libuv.pc])
    ])
    AC_CONFIG_FILES([Makefile])

Therefore, Makefile is generated after libuv.pc. Variables from
Makefile don't affect config.status and libuv.pc.

PR-URL: https://github.com/libuv/libuv/pull/841
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

Makefile.am

@@ -45,7 +45,6 @@ include_HEADERS += include/uv-win.h include/tree.h
 AM_CPPFLAGS += -I$(top_srcdir)/src/win \
                -DWIN32_LEAN_AND_MEAN \
                -D_WIN32_WINNT=0x0600
-LIBS += -lws2_32 -lpsapi -liphlpapi -lshell32 -luserenv
 libuv_la_SOURCES += src/win/async.c \
                     src/win/atomicops-inl.h \
                     src/win/core.c \

configure.ac

@@ -58,6 +58,9 @@ AM_CONDITIONAL([NETBSD],   [AS_CASE([$host_os],[netbsd*],       [true], [false])
 AM_CONDITIONAL([OPENBSD],  [AS_CASE([$host_os],[openbsd*],      [true], [false])])
 AM_CONDITIONAL([SUNOS],    [AS_CASE([$host_os],[solaris*],      [true], [false])])
 AM_CONDITIONAL([WINNT],    [AS_CASE([$host_os],[mingw*],        [true], [false])])
+AS_CASE([$host_os],[mingw*], [
+    LIBS="$LIBS -lws2_32 -lpsapi -liphlpapi -lshell32 -luserenv"
+])
 AC_CHECK_HEADERS([sys/ahafs_evProds.h])
 AC_CHECK_PROG(PKG_CONFIG, pkg-config, yes)
 AM_CONDITIONAL([HAVE_PKG_CONFIG], [test "x$PKG_CONFIG" != "x"])