configure: Cygwin requires _XOPEN_SOURCE >= 700 to prototype strndup()
configure: Cygwin requires _XOPEN_SOURCE >= 700 to prototype strndup()

Adjust definition of _XOPEN_SOURCE appropriately for use of strndup() added with
commit da3a47d6

Signed-off-by: Jon TURNEY <45846cba2ea093796a8b1c25d70d679d8107179f@dronecode.org.uk>

zz85/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zeux/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,dellis1972/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,dellis1972/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,djreep81/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,bkaradzic/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,dellis1972/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,benaadams/glsl-optimizer,zz85/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,zz85/glsl-optimizer,dellis1972/glsl-optimizer,wolf96/glsl-optimizer,zz85/glsl-optimizer,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,tokyovigilante/glsl-optimizer,bkaradzic/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer