build: use DYLD_LIBRARY_PATH on macOS

cf. https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/DynamicLibraryUsageGuidelines.html#//apple_ref/doc/uid/TP40001928-SW12

"""
When the library name is a filename (that is, when it doesn’t include
directory names), the dynamic loader searches for the library in several
locations until it finds it, in the following order:

 - $LD_LIBRARY_PATH
 - $DYLD_LIBRARY_PATH
 - The process’s working directory
 - $DYLD_FALLBACK_LIBRARY_PATH

When the library name contains at least one directory name, that is,
when the name is a pathname (relative or fully qualified), the dynamic
loader searches for the library in the following order:

 - $DYLD_LIBRARY_PATH using the filename
 - The given pathname
 - $DYLD_FALLBACK_LIBRARY_PATH using the filename
 """

Conclusion: DYLD_LIBRARY_PATH on macOS behave like LD_LIBRARY_PATH on
Linux but LD_LIBRARY_PATH on macOS has a different behavior than
LD_LIBRARY_PATH on Linux, so DYLD_LIBRARY_PATH should be used on macOS.
Specially since we can't really control if the library name will include
a path or not (relative or absolute).