configure: don't enable RTMP if the lib detect fails

librtmp is often statically linked and using sub dependencies like
OpenSSL, so we need to make sure we can actually link with it properly
before enabling it. Otherwise we easily end up trying to link with a
RTMP lib that fails.

configure.ac

@@ -2065,6 +2065,7 @@ if test X"$OPT_LIBRTMP" != Xno; then
     ;;
   off)
     dnl no --with-librtmp option given, just check default places
+    LIB_RTMP="-lrtmp"
     ;;
   *)
     dnl use the given --with-librtmp spot
@@ -2083,13 +2084,19 @@ if test X"$OPT_LIBRTMP" != Xno; then
   CPPFLAGS="$CPPFLAGS $CPP_RTMP"
   LIBS="$LIBS $LIB_RTMP"
 
-  AC_CHECK_LIB(rtmp, RTMP_Init)
-
-  AC_CHECK_HEADERS(librtmp/rtmp.h,
-    curl_rtmp_msg="enabled (librtmp)"
-    LIBRTMP_ENABLED=1
-    AC_DEFINE(USE_LIBRTMP, 1, [if librtmp is in use])
-    AC_SUBST(USE_LIBRTMP, [1])
+  AC_CHECK_LIB(rtmp, RTMP_Init,
+    [
+     AC_CHECK_HEADERS(librtmp/rtmp.h,
+        curl_rtmp_msg="enabled (librtmp)"
+        LIBRTMP_ENABLED=1
+        AC_DEFINE(USE_LIBRTMP, 1, [if librtmp is in use])
+        AC_SUBST(USE_LIBRTMP, [1])
+     )
+    ],
+      dnl not found, revert back to clean variables
+      LDFLAGS=$CLEANLDFLAGS
+      CPPFLAGS=$CLEANCPPFLAGS
+      LIBS=$CLEANLIBS
   )
 
   if test X"$OPT_LIBRTMP" != Xoff &&
@@ -2097,12 +2104,6 @@ if test X"$OPT_LIBRTMP" != Xno; then
     AC_MSG_ERROR([librtmp libs and/or directories were not found where specified!])
   fi
 
-  if test "$LIBRTMP_ENABLED" != "1"; then
-    dnl no librtmp, revert back to clean variables
-    LDFLAGS=$CLEANLDFLAGS
-    CPPFLAGS=$CLEANCPPFLAGS
-    LIBS=$CLEANLIBS
-  fi
 fi
 
 dnl **********************************************************************