Bug: https://bugs.gentoo.org/625930
Backported from https://github.com/mean00/avidemux2/commit/c5edc6cf7b3768ac1409c6cef69ab5980a86596a

diff -Naur a/cmake/admCheckX264.cmake b/cmake/admCheckX264.cmake
--- a/cmake/admCheckX264.cmake	2017-04-28 05:22:27.000000000 -0400
+++ b/cmake/admCheckX264.cmake	2017-10-01 01:38:33.249996381 -0400
@@ -20,9 +20,9 @@
 				ELSE (x264_version LESS 67)
                                         IF (x264_version GREATER 73)
 					        FIND_HEADER_AND_LIB(X264 x264.h x264 x264_encoder_open_${x264_version})
+                                        ELSE (x264_version GREATER 73)
+					        FIND_HEADER_AND_LIB(X264 x264.h x264 x264_encoder_open)
                                         ENDIF (x264_version GREATER 73)
-				ELSE (x264_version LESS 67)
-					FIND_HEADER_AND_LIB(X264 x264.h x264 x264_encoder_open)
 				ENDIF (x264_version LESS 67)
                                 IF(X264_FOUND)
                                         SET(USE_X264 True CACHE BOOL "")