LD_LIBRARY_PATH is never used by anything on macOS (and it's not used by the compiler to find libraries on other platforms either). Use the compiler's -Fl command line option to make it search a custom directory.
However, there is a bug in FPC 3.0.4 (and 3.0.4a) that breaks -Fl in combination with a non-empty -XR command line parameter, and on macOS 10.14 the default fpc.cfg configuration file contains this parameter because some files are no longer installed in the same location as on previous OS versions.
The easiest way to work around it is probably to pass "-XR -WM10.8" on the command line. This will
a) clear the custom sysroot (-XR parameter), so -Fl parameters work again
b) set the minimum required target version to macOS 10.8. In that case, the files that have been moved on macOS 10.14 are not required, which is why setting the -XR parameter to empty again won't cause troubles
You don't need an explicit -Fl/usr/local/lib in this case, because that directory is searched by default (when a custom sysroot is specified with -XR, the usr/local/lib directory under this sysroot is searched by default instead).