Warnings from NumericsShims.h when building on Linux

[Ed Connell]

Hi, I can successfully build on Linux but I get the following warnings from NumericsShims.h:
It would be great to have a clean build :)

Thanks, Ed

$ swift build
In file included from /usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/NumericsShims.c:19:
/usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/include/NumericsShims.h:137:10: warning: implicit declaration of function 'lgammaf_r' is invalid in C99 [-Wimplicit-function-declaratio
  return lgammaf_r(x, signp);
         ^
 warning: implicit declaration of function 'lgamma_r' is invalid in C99 [-Wimplicit-function-declaration]
  return lgamma_r(x, signp);

2 warnings generated.
<module-includes>:1:10: note: in file included from <module-includes>:1:
#include "/usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/include/NumericsShims.h"
         ^
/usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/include/NumericsShims.h:137:10: warning: implicit declaration of function 'lgammaf_r' is invalid in C99
  return lgammaf_r(x, signp);
         ^
<module-includes>:1:10: note: in file included from <module-includes>:1:
#include "/usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/include/NumericsShims.h"
         ^
/usr/local/google/home/ewconnell/swiftrt/.build/checkouts/swift-numerics/Sources/NumericsShims/include/NumericsShims.h:259:10: warning: implicit declaration of function 'lgamma_r' is invalid in C99
  return lgamma_r(x, signp);
         ^