library: Change linux version

Added function procps_linux_version() which used to be an
exported integer instead.  Also changed the method of obtaining
the linux version (more correctly the os release) to use a specific
procfs entry. This works for both Linux and FreeBSD.

proc/procps.h

@@ -78,21 +78,6 @@
 #define OBSOLETE
 #endif
 
-#if ( __GNUC__ == 3 && __GNUC_MINOR__ > 1 ) || __GNUC__ > 3
-// Tells gcc that function is library-internal;
-// so no need to do dynamic linking at run-time.
-// This might work with slightly older compilers too.
-#define HIDDEN __attribute__((visibility("hidden")))
-// The opposite, in case -fvisibility=hidden used
-#define EXPORT __attribute__((visibility("default")))
-// Tell g++ that a function won't throw exceptions.
-#define NOTHROW __attribute__((__nothrow__))
-#else
-#define HIDDEN
-#define EXPORT
-#define NOTHROW
-#endif
-
 // Like HIDDEN, but for an alias that gets created.
 // In gcc-3.2 there is an alias+hidden conflict.
 // Many will have patched this bug, but oh well.