Limit exported symbols

This greatly reduces the number of exported symbols by limiting the
namespace of exported symbols to {varnam,varray,vm}_ thus avoiding the
export of all the `_cg` symbols from go.

Before:

$ readelf -s /tmp/a/usr/lib/libgovarnam.so.1.9.0  | grep -v " UND " | wc -l
248

After:

$ readelf -s /usr/lib/libgovarnam.so.1.9.0  | grep -v " UND " | wc -l
52

With this we can make sure applications don't link against accidentally
exported symbols and break on library upgrades without us being able to
notice. It also allows us to notice when symbols go missing and we hence
need to bump the ABI version.

Makefile

@@ -17,12 +17,13 @@ UNAME := $(shell uname)
 SED := sed -i
 LIB_NAME := libgovarnam.so
 SO_NAME := $(shell (echo $(VERSION) | cut -d. -f1))
+CURDIR := $(shell pwd)
 
 ifeq ($(UNAME), Darwin)
   SED := sed -i ""
   LIB_NAME = libgovarnam.dylib
 else
-  EXT_LDFLAGS = -extldflags -Wl,-soname,$(LIB_NAME).$(SO_NAME)
+  EXT_LDFLAGS = -extldflags "-Wl,-soname,$(LIB_NAME).$(SO_NAME),--version-script,$(CURDIR)/govarnam.syms"
 endif
 
 VERSION_STAMP_LDFLAGS := -X 'github.com/varnamproject/govarnam/govarnam.BuildString=${BUILDSTR}' -X 'github.com/varnamproject/govarnam/govarnam.VersionString=${VERSION}' $(EXT_LDFLAGS)

govarnam.syms

@@ -0,0 +1,9 @@
+{
+        global:
+                varnam_*;
+                varray_*;
+                vm_*;
+        local:
+                *;
+};
+