Add pkg-config file for libjq

pkg-config is a common interface to resolve build details. It will
become easier to incorporate libjq in other projects by providing this
interface.

Makefile.am

@@ -175,6 +175,7 @@ DOC_FILES = docs/content docs/public docs/templates docs/site.yml       \
 EXTRA_DIST = $(DOC_FILES) $(man_MANS) $(TESTS) $(TEST_LOG_COMPILER)     \
         jq.1.prebuilt jq.spec src/lexer.c src/lexer.h src/parser.c      \
         src/parser.h src/version.h src/builtin.jq scripts/version       \
+        libjq.pc                                                        \
         tests/base64.test tests/jq-f-test.sh tests/jq.test              \
         tests/modules/a.jq tests/modules/b/b.jq tests/modules/c/c.jq    \
         tests/modules/c/d.jq tests/modules/data.json                    \
@@ -194,6 +195,9 @@ EXTRA_DIST = $(DOC_FILES) $(man_MANS) $(TESTS) $(TEST_LOG_COMPILER)     \
 docdir = ${datadir}/doc/${PACKAGE}
 dist_doc_DATA = README.md COPYING AUTHORS README
 
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = libjq.pc
+
 RELEASE ?= 1
 rpm: dist jq.spec
 	@echo "Packaging jq as an RPM ..."