[build] don't force RUNTIME targets in CMAKE_INSTALL_BINDIR

It is implied by the target type:

> For regular executables, static libraries and shared libraries, the
> DESTINATION argument is not required. For these target types, when
> DESTINATION is omitted, a default destination will be taken from the
> appropriate variable from GNUInstallDirs, or set to a built-in default value
> if that variable is not defined.

https://cmake.org/cmake/help/v3.14/command/install.html#targets

CMakeLists.txt

@@ -1148,7 +1148,13 @@ endmacro()
 
 macro(srt_add_program name)
 	srt_add_program_dont_install(${name} ${ARGN})
-	install(TARGETS ${name} RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+	if(NOT NEED_DESTINATION)
+		install(TARGETS ${name} RUNTIME)
+	elseif (DEFINED CMAKE_INSTALL_BINDIR)
+		install(TARGETS ${name} RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+	else()
+		message(WARNING "No location to install program ${name}")
+	endif()
 endmacro()
 
 macro(srt_make_application name)
@@ -1187,7 +1193,13 @@ endmacro()
 macro(srt_add_application name) # ARGN=sources...
 	srt_add_program(${name} apps/${name}.cpp ${ARGN})
 	srt_make_application(${name})
-	install(TARGETS ${name} RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+	if(NOT NEED_DESTINATION)
+		install(TARGETS ${name} RUNTIME)
+	elseif (DEFINED CMAKE_INSTALL_BINDIR)
+		install(TARGETS ${name} RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+	else()
+		message(WARNING "No location to install program ${name}")
+	endif()
 endmacro()
 
 ## FIXME: transmitmedia.cpp does not build on OpenBSD