interface/vcos/pthreads/CMakeLists.txt: fix build with libexecinfo

interface/vcos/glibc/vcos_backtrace.c uses execinfo.h which can be
provided by libexecinfo on uclibc or musl so link with it if needed to
avoid the following build failure:

/home/peko/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/10.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: ../../build/lib/libvcos.so: undefined reference to `backtrace_symbols'

Fixes:
 - http://autobuild.buildroot.org/results/93d3b8cc2ac5dfa9d4b44946c0b4d8171e8f52a1

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

interface/vcos/pthreads/CMakeLists.txt

@@ -33,12 +33,14 @@ set (SOURCES
    ../generic/vcos_generic_blockpool.c
 )
 
+find_library (EXECINFO_LIBRARY execinfo)
+
 if (VCOS_PTHREADS_BUILD_SHARED)
    add_library (vcos SHARED ${SOURCES})
-   target_link_libraries (vcos pthread dl rt)
+   target_link_libraries (vcos pthread dl rt $<$<BOOL:${EXECINFO_LIBRARY}>:${EXECINFO_LIBRARY}>)
 else ()
    add_library (vcos ${SOURCES})
-   target_link_libraries (vcos pthread rt)
+   target_link_libraries (vcos pthread rt $<$<BOOL:${EXECINFO_LIBRARY}>:${EXECINFO_LIBRARY}>)
 endif ()