libwebsockets

openssl: detect apis correctly when pthreads not available

Since version 3.1.0 and commit aa4143aebdb59f4f21094781fb3b6ab4649643a6,
-pthread is unconditionally added to CMAKE_REQUIRED_FLAGS even if
pthread.h is not found, this will result in a build failure with openssl
if the toolchain doesn't support threads:

[  5%] Building C object CMakeFiles/websockets_shared.dir/lib/core/lws_dll2.c.o
In file included from /home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/include/libwebsockets.h:570,
                 from /home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/lib/core/private.h:130,
                 from /home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/lib/core/lws_dll2.c:22:
/home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/include/libwebsockets/lws-genhash.h:79:18: error: field 'ctx' has incomplete type
         HMAC_CTX ctx;
                  ^~~

This build failure is raised because openssl functions are not correcly
detected:

Determining if the function SSL_CTX_set1_param exists failed with the following output:
Change Dir: /home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_06946/fast && make[1]: Entering directory '/home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_06946.dir/build.make CMakeFiles/cmTC_06946.dir/build
make[2]: Entering directory '/home/buildroot/autobuild/instance-2/output-1/build/libwebsockets-3.2.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_06946.dir/CheckFunctionExists.c.o
/home/buildroot/autobuild/instance-2/output-1/host/bin/arm-linux-gcc --sysroot=/home/buildroot/autobuild/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabihf/sysroot -DKEYWORD=__inline  -Wall -Wsign-compare -Wuninitialized -Werror  -Wundef  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os  -DCHECK_FUNCTION_EXISTS=SSL_CTX_set1_param -pthread  -DNDEBUG   -o CMakeFiles/cmTC_06946.dir/CheckFunctionExists.c.o   -c /home/buildroot/autobuild/instance-2/output-1/host/share/cmake-3.15/Modules/CheckFunctionExists.c
Linking C executable cmTC_06946
/home/buildroot/autobuild/instance-2/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_06946.dir/link.txt --verbose=1
/home/buildroot/autobuild/instance-2/output-1/host/bin/arm-linux-gcc --sysroot=/home/buildroot/autobuild/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabihf/sysroot -Wall -Wsign-compare -Wuninitialized -Werror  -Wundef  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os  -DCHECK_FUNCTION_EXISTS=SSL_CTX_set1_param -pthread  -DNDEBUG    CMakeFiles/cmTC_06946.dir/CheckFunctionExists.c.o  -o cmTC_06946 /home/buildroot/autobuild/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/lib/libssl.so /home/buildroot/autobuild/instance-2/output-1/host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/lib/libcrypto.so -lssl -lcrypto -lm -lcap
/home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabihf/8.3.0/../../../../arm-buildroot-linux-uclibcgnueabihf/bin/ld: cannot find -lpthread
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_06946.dir/build.make:88: recipe for target 'cmTC_06946' failed

Fixes:
 - http://autobuild.buildroot.org/results/6186b4718db285edadf7203d00ed72f8d76a31e4

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>