Makefile: use pkg-config to get libelf flags

libbpf uses libelf symbols internally, so it depends on it and needs to
link against it.

Upstream this was fixed by linking directory to libelf:

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=89dedaef49d36adc2bb5e7e4c38b52fa3013c7c8

For this project, intended to be used as a general library, try first
to use pkg-config instead, and only fallback to just passing -lelf.
This will be useful for cross compiling, among other cases.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>

README

@@ -19,6 +19,13 @@ successful.
 
 Build
 =====
+libelf is an internal dependency of libbpf and thus it is required to link
+against and must be installed on the system for applications to work.
+pkg-config is used by default to find libelf, and the program called can be
+overridden with PKG_CONFIG.
+If using pkg-config at build time is not desired, it can be disabled by setting
+NO_PKG_CONFIG=1 when calling make.
+
 To build both static libbpf.a and shared libbpf.so:
   cd src
   make
@@ -29,3 +36,10 @@ root/:
   cd src
   mkdir build root
   BUILD_STATIC_ONLY=y OBJDIR=build DESTDIR=root make install
+
+To build both static libbpf.a and shared libbpf.so against a custom libelf
+dependency installed in /build/root/ and install them together with libbpf
+headers in a build directory /build/root/:
+
+  cd src
+  PKG_CONFIG_PATH=/build/root/lib64/pkgconfig DESTDIR=/build/root make install