add boolean parameter use_system for searching boost

This boolean parameter is added to check_and_set_roots() and detect_lib_dirs()
when true it will first search the compiler library directories before checking
the standard lib directories. This is set to false when using BOOST_ROOT and
set to true when useing other system directories like /usr/local

Also simplify using a set to remove duplicate lib files

Also remove the part where you search the Cellar in homebrew, this is
unnescessary now that symlinks can be followed it should find boost
when searching /usr/local so no need to search the Cellar

mesonbuild/dependencies/boost.py

@@ -382,7 +382,7 @@ class BoostDependency(SystemDependency):
         # Finally, look for paths from .pc files and from searching the filesystem
         self.detect_roots()
 
-    def check_and_set_roots(self, roots: T.List[Path]) -> None:
+    def check_and_set_roots(self, roots: T.List[Path], use_system: bool) -> None:
         roots = list(mesonlib.OrderedSet(roots))
         for j in roots:
             #   1. Look for the boost headers (boost/version.hpp)
@@ -394,7 +394,7 @@ class BoostDependency(SystemDependency):
             if not inc_dirs:
                 continue
 
-            lib_dirs = self.detect_lib_dirs(j)
+            lib_dirs = self.detect_lib_dirs(j, use_system)
             self.is_found = self.run_check(inc_dirs, lib_dirs)
             if self.is_found:
                 self.boost_root = j
@@ -436,7 +436,7 @@ class BoostDependency(SystemDependency):
         if paths and any([not x.is_absolute() for x in paths]):
             raise DependencyException('boost_root path given in machine file must be absolute')
 
-        self.check_and_set_roots(paths)
+        self.check_and_set_roots(paths, use_system=False)
 
     def run_check(self, inc_dirs: T.List[BoostIncludeDir], lib_dirs: T.List[Path]) -> bool:
         mlog.debug('  - potential library dirs: {}'.format([x.as_posix() for x in lib_dirs]))
@@ -532,16 +532,22 @@ class BoostDependency(SystemDependency):
         candidates = [x for x in candidates if x.exists()]
         return [self._include_dir_from_version_header(x) for x in candidates]
 
-    def detect_lib_dirs(self, root: Path) -> T.List[Path]:
+    def detect_lib_dirs(self, root: Path, use_system: bool) -> T.List[Path]:
         # First check the system include paths. Only consider those within the
         # given root path
-        system_dirs_t = self.clib_compiler.get_library_dirs(self.env)
-        system_dirs = [Path(x) for x in system_dirs_t]
-        system_dirs = [x.resolve() for x in system_dirs if x.exists()]
-        system_dirs = [x for x in system_dirs if mesonlib.path_is_in_root(x, root)]
-        system_dirs = list(mesonlib.OrderedSet(system_dirs))
 
-        # In addition to the system include paths, also look in "lib"
+        if use_system:
+            system_dirs_t = self.clib_compiler.get_library_dirs(self.env)
+            system_dirs = [Path(x) for x in system_dirs_t]
+            system_dirs = [x.resolve() for x in system_dirs if x.exists()]
+            system_dirs = [x for x in system_dirs if mesonlib.path_is_in_root(x, root)]
+            system_dirs = list(mesonlib.OrderedSet(system_dirs))
+
+            if system_dirs:
+                return system_dirs
+
+        # No system include paths were found --> fall back to manually looking
+        # for library dirs in root
         dirs = []     # type: T.List[Path]
         subdirs = []  # type: T.List[Path]
         for i in root.iterdir():
@@ -557,7 +563,7 @@ class BoostDependency(SystemDependency):
         # Filter out paths that don't match the target arch to avoid finding
         # the wrong libraries. See https://github.com/mesonbuild/meson/issues/7110
         if not self.arch:
-            return system_dirs + dirs + subdirs
+            return dirs + subdirs
 
         arch_list_32 = ['32', 'i386']
         arch_list_64 = ['64']
@@ -611,19 +617,16 @@ class BoostDependency(SystemDependency):
         return libs
 
     def detect_libraries(self, libdir: Path) -> T.List[BoostLibraryFile]:
-        libs = set()  # type: T.Set[Path]
+        libs = set()  # type: T.Set[BoostLibraryFile]
         for i in libdir.iterdir():
             if not i.is_file():
                 continue
             if not any([i.name.startswith(x) for x in ['libboost_', 'boost_']]):
                 continue
 
-            libs.add(i.resolve())
+            libs.add(BoostLibraryFile(i.resolve()))
 
-        # Remove duplicate libraries caused by resolving symlinks
-        blibs = [BoostLibraryFile(i) for i in libs] # type: T.List[BoostLibraryFile]
-
-        return [x for x in blibs if x.is_boost()]  # Filter out no boost libraries
+        return [x for x in libs if x.is_boost()]  # Filter out no boost libraries
 
     def detect_split_root(self, inc_dir: Path, lib_dir: Path) -> None:
         boost_inc_dir = None
@@ -679,11 +682,6 @@ class BoostDependency(SystemDependency):
         else:
             tmp = []  # type: T.List[Path]
 
-            # Homebrew
-            brew_boost = Path('/usr/local/Cellar/boost')
-            if brew_boost.is_dir():
-                tmp += [x for x in brew_boost.iterdir()]
-
             # Add some default system paths
             tmp += [Path('/opt/local')]
             tmp += [Path('/usr/local/opt/boost')]
@@ -695,7 +693,7 @@ class BoostDependency(SystemDependency):
             tmp = [x.resolve() for x in tmp]
             roots += tmp
 
-        self.check_and_set_roots(roots)
+        self.check_and_set_roots(roots, use_system=True)
 
     def log_details(self) -> str:
         res = ''