Add a --disable-opencl configure option to make it possible to override detection of opencl and build without GPU mining support.

configure.ac

@@ -51,6 +51,11 @@ case $target in
 	;;
 esac
 
+AC_ARG_ENABLE([opencl],
+	[AC_HELP_STRING([--disable-opencl],[Override detection and disable building with opencl])],
+	[opencl=$enableval]
+	)
+if test "x$opencl" != xno; then
 	# Check for OpenCL (the long way needed on mingw32 due to calling conventions)
 	AC_MSG_CHECKING([for OpenCL])
 	SAVED_LIBS=$LIBS
@@ -71,6 +76,7 @@ AC_LINK_IFELSE(
 	[AC_MSG_RESULT(no)
 	found_opencl=0])
 	LIBS=$SAVED_LIBS
+fi
 
 AC_CHECK_LIB(pthread, pthread_create, ,
         AC_MSG_ERROR([Could not find pthread library - please install libpthread]))
@@ -201,8 +207,12 @@ AC_OUTPUT
 
 echo ''
 
+if test "x$opencl" != xno; then
 	if test $found_opencl = 1; then
 		echo OpenCL: FOUND. GPU mining support enabled.
 	else
 		echo OpenCL: NOT FOUND. GPU mining support DISABLED.
 	fi
+else
+	echo OpenCL: Detection overrided. GPU mining support DISABLED.
+fi