Disable install/installcheck targets in doc/Makefile

Remove inclusion of 'pg_config --pgxs' makefile because it is not
required to create the documentation. This makefile was designed to be
included in top-level extension makefile. Inclusion in the doc/Makefile
results into some unexpected behaviour when using 'install' or 'all' rules.

Remove gmake -C doc install in .travis.yml because this target didn't
do anything. I guess that the installation of the manual should be
implemented in the top-level extension Makefile file, not in the
doc/Makefile.

Author: vitcpp

Diff for: .travis.yml

@@ -40,4 +40,3 @@ script:
   - pg_virtualenv make USE_HEALPIX=0 crushtest
   - if test -s regression.diffs; then cat regression.diffs; exit 1; fi
   - make -C doc
-  - sudo make -C doc install

Diff for: doc/Makefile

@@ -11,35 +11,20 @@ $(error PGSPHERE_VERSION is not set)
 endif
 endif
 
-USE_PGXS = 1
-
-ifdef USE_PGXS
-PG_CONFIG = pg_config
-PGXS := $(shell $(PG_CONFIG) --pgxs)
-include $(PGXS)
-else
-subdir = contrib/pg_sphere/doc
-top_builddir = ../../..
-include $(top_builddir)/src/Makefile.global
-include $(top_srcdir)/contrib/contrib-global.mk
-endif
-
-.SECONDARY:
-
 ifndef FOP
 FOP = fop
 endif
 
 ifdef XMLLINT
 XMLLINT := $(XMLLINT) --nonet
 else
-XMLLINT = $(missing) xmllint
+XMLLINT = xmllint --nonet
 endif
 
 ifdef XSLTPROC
 XSLTPROC := $(XSLTPROC) --nonet
 else
-XSLTPROC = $(missing) xsltproc
+XSLTPROC = xsltproc --nonet
 endif
 
 override XSLTPROCFLAGS += \