ARROW-1419: [GLib] Suppress sign-conversion warnings
ARROW-1419: [GLib] Suppress sign-conversion warnings

Apache Arrow C++ uses int as result type for expression that uses
size_t. It causes sign-conversion warning but the coding style is
expected.

Example:

    .../arrow/buffer.h:296:41: warning:
          implicit conversion changes signedness: 'unsigned long' to 'int64_t'
          (aka 'long') [-Wsign-conversion]
      int64_t length() const { return size_ / sizeof(T); }
                               ~~~~~~ ~~~~~~^~~~~~~~~~~

Author: Kouhei Sutou <kou@clear-code.com>

Closes #999 from kou/glib-suppress-warning-on-clang and squashes the following commits:

397490e9 [Kouhei Sutou] [GLib] Suppress sign-conversion warnings

xhochy/arrow,NonVolatileComputing/arrow,majetideepak/arrow,itaiin/arrow,wesm/arrow,dremio/arrow,wesm/arrow,dremio/arrow,dremio/arrow,wagavulin/arrow,tebeka/arrow,cpcloud/arrow,xhochy/arrow,cpcloud/arrow,cpcloud/arrow,cpcloud/arrow,pcmoritz/arrow,dremio/arrow,dremio/arrow,apache/arrow,majetideepak/arrow,renesugar/arrow,siddharthteotia/arrow,kou/arrow,renesugar/arrow,NonVolatileComputing/arrow,apache/arrow,yufeldman/arrow,kou/arrow,laurentgo/arrow,wesm/arrow,tebeka/arrow,pcmoritz/arrow,majetideepak/arrow,laurentgo/arrow,icexelloss/arrow,tebeka/arrow,icexelloss/arrow,cpcloud/arrow,cpcloud/arrow,laurentgo/arrow,siddharthteotia/arrow,renesugar/arrow,wagavulin/arrow,cpcloud/arrow,itaiin/arrow,dremio/arrow,yufeldman/arrow,cpcloud/arrow,wagavulin/arrow,renesugar/arrow,apache/arrow,pcmoritz/arrow,siddharthteotia/arrow,renesugar/arrow,wesm/arrow,apache/arrow,yufeldman/arrow,kou/arrow,majetideepak/arrow,kou/arrow,cpcloud/arrow,cpcloud/arrow,icexelloss/arrow,apache/arrow,pcmoritz/arrow,xhochy/arrow,dremio/arrow,yufeldman/arrow,kou/arrow,renesugar/arrow,majetideepak/arrow,itaiin/arrow,itaiin/arrow,icexelloss/arrow,itaiin/arrow,cpcloud/arrow,wagavulin/arrow,yufeldman/arrow,laurentgo/arrow,itaiin/arrow,icexelloss/arrow,apache/arrow,siddharthteotia/arrow,NonVolatileComputing/arrow,apache/arrow,wagavulin/arrow,laurentgo/arrow,wesm/arrow,dremio/arrow,tebeka/arrow,pcmoritz/arrow,wesm/arrow,icexelloss/arrow,xhochy/arrow,wesm/arrow,itaiin/arrow,icexelloss/arrow,yufeldman/arrow,icexelloss/arrow,apache/arrow,NonVolatileComputing/arrow,renesugar/arrow,pcmoritz/arrow,tebeka/arrow,majetideepak/arrow,renesugar/arrow,wagavulin/arrow,apache/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,cpcloud/arrow,itaiin/arrow,wesm/arrow,yufeldman/arrow,renesugar/arrow,itaiin/arrow,wagavulin/arrow,kou/arrow,apache/arrow,laurentgo/arrow,xhochy/arrow,NonVolatileComputing/arrow,pcmoritz/arrow,siddharthteotia/arrow,majetideepak/arrow,wagavulin/arrow,kou/arrow,icexelloss/arrow,NonVolatileComputing/arrow,renesugar/arrow,yufeldman/arrow,itaiin/arrow,yufeldman/arrow,wagavulin/arrow,majetideepak/arrow,wesm/arrow,siddharthteotia/arrow,wagavulin/arrow,icexelloss/arrow,laurentgo/arrow,apache/arrow,xhochy/arrow,siddharthteotia/arrow,renesugar/arrow,xhochy/arrow,xhochy/arrow,tebeka/arrow,pcmoritz/arrow,majetideepak/arrow,laurentgo/arrow,tebeka/arrow,majetideepak/arrow,laurentgo/arrow,xhochy/arrow,majetideepak/arrow,renesugar/arrow,laurentgo/arrow,dremio/arrow,pcmoritz/arrow,siddharthteotia/arrow,majetideepak/arrow,wesm/arrow,cpcloud/arrow,pcmoritz/arrow,itaiin/arrow,NonVolatileComputing/arrow,laurentgo/arrow,yufeldman/arrow,icexelloss/arrow,cpcloud/arrow,pcmoritz/arrow,NonVolatileComputing/arrow,siddharthteotia/arrow,itaiin/arrow,dremio/arrow,wagavulin/arrow,NonVolatileComputing/arrow,wesm/arrow,laurentgo/arrow,dremio/arrow,kou/arrow,renesugar/arrow,majetideepak/arrow,apache/arrow,wagavulin/arrow,NonVolatileComputing/arrow,majetideepak/arrow,xhochy/arrow,kou/arrow,majetideepak/arrow,itaiin/arrow,itaiin/arrow,laurentgo/arrow,icexelloss/arrow,itaiin/arrow,siddharthteotia/arrow,kou/arrow,xhochy/arrow,yufeldman/arrow,renesugar/arrow,tebeka/arrow,laurentgo/arrow,xhochy/arrow,kou/arrow,tebeka/arrow,pcmoritz/arrow,xhochy/arrow,laurentgo/arrow,apache/arrow,pcmoritz/arrow,xhochy/arrow,icexelloss/arrow,xhochy/arrow,tebeka/arrow,apache/arrow,pcmoritz/arrow,kou/arrow,wesm/arrow,NonVolatileComputing/arrow,siddharthteotia/arrow,wesm/arrow,pcmoritz/arrow,kou/arrow,renesugar/arrow