configure.ac: revert bad nghttp2 library detection improvements

This reverts commit b4b34db, 6737533 and 29c7cf7.

The logic is now back to assuming that the nghttp2 lib is called nghttp2 and
nothing else.

Reported-by: Rui Pinheiro
Reported-by: Alex Crichton
Fixes curl#7514
Closes curl#7515

Author: bagder

configure.ac

@@ -2541,11 +2541,10 @@ if test X"$want_h2" != Xno; then
     LDFLAGS="$LDFLAGS $LD_H2"
     CPPFLAGS="$CPPFLAGS $CPP_H2"
     LIBS="$LIB_H2 $LIBS"
-    LIB_H2_NAME=`echo $LIB_H2 | $SED -ne 's/.*-l *\(nghttp2[^ ]*\).*/\1/p'`
 
     # use nghttp2_session_set_local_window_size to require nghttp2
     # >= 1.12.0
-    AC_CHECK_LIB($LIB_H2_NAME, nghttp2_session_set_local_window_size,
+    AC_CHECK_LIB(nghttp2, nghttp2_session_set_local_window_size,
       [
        AC_CHECK_HEADERS(nghttp2/nghttp2.h,
           curl_h2_msg="enabled (nghttp2)"