Fix linking on FreeBSD (#138)

In CMakeLists.txt, calling link_directories(${PROJECT_SOURCE_DIR}/lib)
for FreeBSD builds causes linking to fail with the error below.

ld: error: undefined symbol: __errno_location
>>> referenced by kmp_alloc.cpp
>>>               kmp_alloc.cpp.o:(kmpc_aligned_malloc) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced by kmp_i18n.cpp
>>>               kmp_i18n.cpp.o:(__kmp_i18n_do_catopen()) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced by kmp_runtime.cpp
>>>               kmp_runtime.cpp.o:(__kmp_allocate_team) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced 16 more times

ld: error: undefined symbol: stderr
>>> referenced by kmp_io.cpp
>>>               kmp_io.cpp.o:(__kmp_vprintf) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced by kmp_io.cpp
>>>               kmp_io.cpp.o:(__kmp_vprintf) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced by kmp_io.cpp
>>>               kmp_io.cpp.o:(__kmp_vprintf) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
>>> referenced 1 more times

ld: error: undefined symbol: pthread_getattr_np
>>> referenced by z_Linux_util.cpp
>>>               z_Linux_util.cpp.o:(__kmp_set_stack_info(int, kmp_info*)) in archive /wrkdirs/usr/ports/biology/iqtree/work/iqtree2-2.2.2.7/lib/libomp.a
c++: error: linker command failed with exit code 1 (use -v to see invocation)

Co-authored-by: Thomas Wong <thomaskf@gmail.com>

CMakeLists.txt

@@ -419,14 +419,14 @@ if (NOT IQTREE_FLAGS MATCHES "single")
     	if(OSX_NATIVE_ARCHITECTURE STREQUAL "arm64")
     		link_directories(${PROJECT_SOURCE_DIR}/libmac_m1)
     	else()
-        	link_directories(${PROJECT_SOURCE_DIR}/libmac)
-        endif()	
-    elseif (UNIX)
-	if (__ARM_NEON)
-		link_directories(${PROJECT_SOURCE_DIR}/liblinux_arm)
-	else()
-		link_directories(${PROJECT_SOURCE_DIR}/lib)
-	endif()
+        link_directories(${PROJECT_SOURCE_DIR}/libmac)
+      endif()	
+    elseif (UNIX AND NOT ${CMAKE_SYSTEM_NAME} STREQUAL "FreeBSD")
+      if (__ARM_NEON)
+        link_directories(${PROJECT_SOURCE_DIR}/liblinux_arm)
+      else()
+        link_directories(${PROJECT_SOURCE_DIR}/lib)
+      endif()
     elseif (WIN32)
         if (BINARY32)
             link_directories(${PROJECT_SOURCE_DIR}/lib32)