build: set OSX_MIN_VERSION to 10.15

This is required to use std::filesystem on macOS as support for it only
landed in the libc++ dylib shipped with 10.15.

See also: https://developer.apple.com/documentation/xcode-release-notes/xcode-11-release-notes

Clang now supports the C++17 <filesystem> library for iOS 13, macOS 10.15, watchOS 6, and tvOS 13.

contrib/guix/libexec/build.sh

@@ -147,7 +147,7 @@ case "$HOST" in
         #
         # After the native packages in depends are built, the ld wrapper should
         # no longer affect our build, as clang would instead reach for
-        # x86_64-apple-darwin18-ld from cctools
+        # x86_64-apple-darwin19-ld from cctools
         ;;
     *) export GUIX_LD_WRAPPER_DISABLE_RPATH=yes ;;
 esac
@@ -423,8 +423,8 @@ mkdir -p "$DISTSRC"
                 find "${DISTNAME}" -print0 \
                     | sort --zero-terminated \
                     | tar --create --no-recursion --mode='u+rw,go+r-w,a+X' --null --files-from=- \
-                    | gzip -9n > "${OUTDIR}/${DISTNAME}-${HOST//x86_64-apple-darwin18/osx64}.tar.gz" \
-                    || ( rm -f "${OUTDIR}/${DISTNAME}-${HOST//x86_64-apple-darwin18/osx64}.tar.gz" && exit 1 )
+                    | gzip -9n > "${OUTDIR}/${DISTNAME}-${HOST//x86_64-apple-darwin19/osx64}.tar.gz" \
+                    || ( rm -f "${OUTDIR}/${DISTNAME}-${HOST//x86_64-apple-darwin19/osx64}.tar.gz" && exit 1 )
                 ;;
         esac
     )  # $DISTSRC/installed