Finally got rid of the error
i ran "make defconfig",
the "make menuconfig" to reselect the options i wanted and all is good now.

after running "make menu config" i went down to "libraries" then selected "libwebsockets"
It went further this time but stopped at another error:

loop.c: In function 'loop_handle_reads_writes':
loop.c:492:22: error: storage size of 'wspoll' isn't known
struct lws_pollfd wspoll;
^
loop.c:496:4: warning: implicit declaration of function 'lws_service_fd' [-Wimplicit-function-declaration]
lws_service_fd(lws_get_context(context->wsi), &wspoll);
^
loop.c:496:19: warning: implicit declaration of function 'lws_get_context' [-Wimplicit-function-declaration]
lws_service_fd(lws_get_context(context->wsi), &wspoll);
^
Makefile:33: recipe for target 'loop.o' failed
make[5]: *** [loop.o] Error 1
make[5]: Leaving directory '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/mosquitto-ssl/mosquitto-1.4.14/src'
Makefile:21: recipe for target 'mosquitto' failed
make[4]: *** [mosquitto] Error 2
make[4]: Leaving directory '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/mosquitto-ssl/mosquitto-1.4.14'
Makefile:223: recipe for target '/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/mosquitto-ssl/mosquitto-1.4.14/.built' failed
make[3]: *** [/root/source/build_dir/target-mipsel_24kc_musl-1.1.16/mosquitto-ssl/mosquitto-1.4.14/.built] Error 2
make[3]: Leaving directory '/root/source/feeds/packages/net/mosquitto'
package/Makefile:105: recipe for target 'package/feeds/packages/mosquitto/compile' failed
make[2]: *** [package/feeds/packages/mosquitto/compile] Error 2
make[2]: Leaving directory '/root/source'
package/Makefile:101: recipe for target '/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/stamp/.package_compile' failed
make[1]: *** [/root/source/staging_dir/target-mipsel_24kc_musl-1.1.16/stamp/.package_compile] Error 2
make[1]: Leaving directory '/root/source'
/root/source/include/toplevel.mk:198: recipe for target 'world' failed
make: *** [world] Error 2
root@7cac80aeef0e:~/source#

@thor42 said in Cross Compile docker libwebsockets.h not found mosquitto:

libwebsockets.h

So far what i undestand the docker image lack the software libwebsockets available from https://github.com/warmcat/libwebsockets

So i can download it to the docker, but unable to get it to compile it yet.