Revert "Use pkg-config to check for libcurl version"

This reverts commit 98387f94acd8c2d335727dd7a6fd900d292a58bf.

Broke generic building.

configure.ac

@@ -124,10 +124,8 @@ AM_CONDITIONAL([HAS_YASM], [test x$has_yasm = xtrue])
 
 PKG_PROG_PKG_CONFIG()
 
-PKG_CHECK_MODULES([LIBCURL], [libcurl >= 7.15.6], [AC_DEFINE([CURL_HAS_SOCKOPT], [1], [Defined if version of curl supports sockopts.])],
-[PKG_CHECK_MODULES([LIBCURL], [libcurl >= 7.10.1], ,[AC_MSG_ERROR([Missing required libcurl dev >= 7.10.1])])])
-AC_SUBST(LIBCURL_CPPFLAGS)
-AC_SUBST(LIBCURL_LIBS)
+LIBCURL_CHECK_CONFIG(, 7.10.1, ,  [AC_MSG_ERROR([Missing required libcurl dev >= 7.10.1])])
+LIBCURL_CHECK_CONFIG(, 7.15.6, [AC_DEFINE([CURL_HAS_SOCKOPT], [1], [Defined if version of curl supports sockopts.])],)
 
 dnl CCAN wants to know a lot of vars.
 # All the configuration checks.  Regrettably, the __attribute__ checks will