[build] Adding pkg-config support

Makefile.am

@@ -1 +1,4 @@
-SUBDIRS = src test
+SUBDIRS = src test
+
+pkgconfigdir = $(libdir)/pkgconfig
+pkgconfig_DATA = libpostal.pc

README.md

@@ -201,12 +201,12 @@ Before you install, make sure you have the following prerequisites:
 
 **On Linux (Debian)**
 ```
-sudo apt-get install libsnappy-dev autoconf automake libtool
+sudo apt-get install libsnappy-dev autoconf automake libtool pkg-config
 ```
 
 **On Mac OSX**
 ```
-sudo brew install snappy autoconf automake libtool
+sudo brew install snappy autoconf automake libtool pkg-config
 ```
 
 Then to install the C library:
@@ -223,6 +223,20 @@ sudo make install
 sudo ldconfig
 ```
 
+libpostal has support for pkg-config, so you can use the pkg-config to print the flags needed to link your program against it:
+
+```
+pkg-config --cflags libpostal         # print compiler flags
+pkg-config --libs libpostal           # print linker flags
+pkg-config --cflags --libs libpostal  # print both
+```
+
+For example:
+
+```
+gcc app.c `pkg-config --cflags --libs libpostal`
+```
+
 Bindings
 --------
 

configure.ac

@@ -51,6 +51,7 @@ AC_FUNC_REALLOC
 AC_CHECK_FUNCS([getcwd gettimeofday memmove memset munmap regcomp setlocale sqrt strdup strndup])
 
 AC_CONFIG_FILES([Makefile
+                 libpostal.pc
                  src/Makefile
                  src/sparkey/Makefile
                  test/Makefile])

libpostal.pc.in

@@ -0,0 +1,11 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: libpostal
+Description: Fast international street address parsing and normalization using statistical NLP.
+Version: @PACKAGE_VERSION@
+Libs: -L${libdir} -lpostal
+Libs.private: @LIBS@
+Cflags: -I${includedir}