spirv-tools: Declare conflict_build with gtest

spirv-tools includes its own embedded googletest and fails to build
when the system-installed googletest (via MacPorts) is present,
due to CMake picking up the external version unintentionally.

Closed: https://trac.macports.org/ticket/72710

Author: judaew

graphics/spirv-tools/Portfile

@@ -4,6 +4,7 @@ PortSystem          1.0
 PortGroup           github 1.0
 PortGroup           cmake 1.1
 PortGroup           legacysupport 1.1
+PortGroup           conflicts_build 1.0
 
 github.setup        KhronosGroup SPIRV-Tools 1.4.328.1 vulkan-sdk-
 github.tarball_from archive
@@ -97,3 +98,5 @@ configure.args-append \
                     -DBUILD_SHARED_LIBS=ON \
                     -DSPIRV_TOOLS_BUILD_STATIC=OFF \
                     -DCMAKE_INSTALL_PREFIX=${prefix}
+
+conflicts_build     gtest