BUG #13497: Build with dtrace fails

The following bug has been logged on the website:

Bug reference: 13497
Logged by: Daniel (dmilith) Dettlaff
Email address: dmilith(at)gmail(dot)com
PostgreSQL version: 9.4.4
Operating system: FreeBSD 10.1
Description:

utils/probes.o: In function `dtrace_dof_init':
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0xd7):
undefined reference to `elf_version'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x10f):
undefined reference to `elf_begin'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x131):
undefined reference to `elf_getshdrstrndx'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x13b):
undefined reference to `elf_nextscn'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x1a2):
undefined reference to `elf_getdata'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x1ce):
undefined reference to `elf_getdata'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x1e1):
undefined reference to `elf_getdata'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x207):
undefined reference to `gelf_getshdr'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x22b):
undefined reference to `elf_strptr'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x24e):
undefined reference to `elf_nextscn'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x5df):
undefined reference to `elf_end'
utils/probes.o: In function `fixsymbol':
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x79b):
undefined reference to `gelf_getsym'
/usr/src/cddl/lib/drti/../../../cddl/contrib/opensolaris/lib/libdtrace/common/drti.c:(.text+0x7d3):
undefined reference to `elf_strptr'
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
Makefile:57: recipe for target 'postgres' failed

To fix this, I needed to add -lelf to LDFLAGS. The build passes.