Relax restrictions on detecting boost dependency

This does two things:
* allows the library files to be symlinks
* searches `lib` and `lib64` in `BOOST_ROOT` even if it finds lib
  directories from the compiler

The first condition is needed for the homebrew on macOS because boost and boost
python are provided in seperate packages and are put together in /usr/local/lib
with symlinks to the library files. The both conditions are needed for high
performace computing environments where dependencies are often provided in
nonstandard directories with symlinks

A test case was added which looks for boost libraries in seperate directories
which have been symlinked to BOOST_ROOT/lib

mesonbuild/dependencies/boost.py

@@ -541,11 +541,7 @@ class BoostDependency(SystemDependency):
         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
+        # In addition to the system include paths, also look in "lib"
         dirs = []     # type: T.List[Path]
         subdirs = []  # type: T.List[Path]
         for i in root.iterdir():
@@ -561,7 +557,7 @@ 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 dirs + subdirs
+            return system_dirs + dirs + subdirs
 
         arch_list_32 = ['32', 'i386']
         arch_list_64 = ['64']
@@ -615,15 +611,19 @@ class BoostDependency(SystemDependency):
         return libs
 
     def detect_libraries(self, libdir: Path) -> T.List[BoostLibraryFile]:
-        libs = []  # type: T.List[BoostLibraryFile]
+        libs = []  # type: T.List[Path]
         for i in libdir.iterdir():
-            if not i.is_file() or i.is_symlink():
+            if not i.is_file():
                 continue
             if not any([i.name.startswith(x) for x in ['libboost_', 'boost_']]):
                 continue
 
-            libs += [BoostLibraryFile(i)]
-        return [x for x in libs if x.is_boost()]  # Filter out no boost libraries
+            libs += [i.resolve()]
+
+        # Remove duplicate libraries caused by resolving symlinks
+        blibs = [BoostLibraryFile(i) for i in set(libs)] # type: T.List[BoostLibraryFile]
+
+        return [x for x in blibs 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

test cases/frameworks/35 boost symlinks/boost/Cellar/boost/0.3.0/include/boost/version.hpp

@@ -0,0 +1,3 @@
+#define BOOST_VERSION 300
+
+#error This is not a real version of boost

test cases/frameworks/35 boost symlinks/boost/include/boost

@@ -0,0 +1 @@
+../Cellar/boost/0.3.0/include/boost

test cases/frameworks/35 boost symlinks/boost/lib/boost_python-vc142-mt-gd-x32-0_3.lib

@@ -0,0 +1 @@
+../Cellar/boost-python/0.3.0/lib/boost_python-vc142-mt-gd-x32-0_3.lib

test cases/frameworks/35 boost symlinks/boost/lib/boost_python-vc142-mt-gd-x64-0_3.lib

@@ -0,0 +1 @@
+../Cellar/boost-python/0.3.0/lib/boost_python-vc142-mt-gd-x64-0_3.lib

test cases/frameworks/35 boost symlinks/boost/lib/boost_regex-vc142-mt-gd-x32-0_3.lib

@@ -0,0 +1 @@
+../Cellar/boost/0.3.0/lib/boost_regex-vc142-mt-gd-x32-0_3.lib

test cases/frameworks/35 boost symlinks/boost/lib/boost_regex-vc142-mt-gd-x64-0_3.lib

@@ -0,0 +1 @@
+../Cellar/boost/0.3.0/lib/boost_regex-vc142-mt-gd-x64-0_3.lib

test cases/frameworks/35 boost symlinks/boost/lib/libboost_python.so.0.3.0

@@ -0,0 +1 @@
+../Cellar/boost-python/0.3.0/lib/libboost_python.so.0.3.0

test cases/frameworks/35 boost symlinks/boost/lib/libboost_regex.so.0.3.0

@@ -0,0 +1 @@
+../Cellar/boost/0.3.0/lib/libboost_regex.so.0.3.0

test cases/frameworks/35 boost symlinks/meson.build

@@ -0,0 +1,6 @@
+project('boosttestsymlinks', 'cpp')
+
+dep = dependency('boost', modules : ['regex', 'python'], required: false)
+
+assert(dep.found(), 'expected to find a fake version of boost')
+assert(dep.version() == '0.3.0', 'expected to find version 0.3.0')

test cases/frameworks/35 boost symlinks/nativefile.ini.in

@@ -0,0 +1,2 @@
+[properties]
+boost_root = '@MESON_TEST_ROOT@/boost'