Fix dockerfile build-dependencies. (#37)

Now that we're using $(shell $(PG_CONFIG) --libs) to have the list of
librairies we depend on, we must align the build dependencies of
pg_auto_failover with the Postgres build dependencies on the platform where
we build.

In debian, that means having build dependencies towards readline
support (that's libedit), pam, zlib, xml, xstl, and selinux.

Also, switch to using `pg_config --ldflags`, which is supposed to be right on all platforms.
From local tests it seems like we still need --pkglibdir at least when
building on debian (make build-test does that in a docker container).

See #34.

Dockerfile

@@ -11,6 +11,12 @@ RUN apt-get update \
     libicu-dev \
     libkrb5-dev \
     libssl-dev \
+    libedit-dev \
+    libpam-dev \
+    zlib1g-dev \
+    libxml2-dev \
+    libxslt1-dev \
+    libselinux1-dev \
     make \
     openssl \
     pipenv \

src/bin/pg_autoctl/Makefile

@@ -31,8 +31,8 @@ CFLAGS += -I $(shell $(PG_CONFIG) --cflags)
 CFLAGS += -Wformat
 CFLAGS += $(COMMON_LIBS)
 
-LIBS  = -L $(shell $(PG_CONFIG) --libdir)
-LIBS += -L $(shell $(PG_CONFIG) --pkglibdir)
+LIBS  = -L $(shell $(PG_CONFIG) --pkglibdir)
+LIBS += $(shell $(PG_CONFIG) --ldflags)
 LIBS += $(shell $(PG_CONFIG) --libs)
 LIBS += -lpq