I see the file in ./Frameworks/ns/src/ns.h, perhaps that framework needs to be built first?
I am sure I can haxor it to work, but would rather not get onto that slippery slope, have been using mate for like 9 years now, and there are some things I would like to fix in 2.0rc4 or at least test to see if they are fixed in master.

Output:

UpLk Prod[textmate] $ ./configure && ninja
rm /Users/gplayer/build/TextMate/Frameworks/SoftwareUpdate/fixtures/secret.pem
bin/gen_build:477: warning: Insecure world writable dir /Applications/microchip/xc16/v1.22/bin in PATH, mode 040777
[1/1] Generate ‘build.ninja’…
bin/gen_build:477: warning: Insecure world writable dir /Applications/microchip/xc16/v1.22/bin in PATH, mode 040777
ninja: no work to do.
ninja: no work to do.
[1/9] Compile ‘Frameworks/OakFoundation/src/OakFoundation.mm’…
FAILED: /Users/gplayer/build/TextMate/Frameworks/OakFoundation/src/OakFoundation.o 
xcrun clang++ -include /Users/gplayer/build/TextMate/Shared/PCH/prelude.mm -c -pipe -fPIC -gdwarf-2 -m64 -mmacosx-version-min=10.9 -funsigned-char -D'NULL_STR="\uFFFF"' -DREST_API='"https://api.textmate.org"' -Wall -Wwrite-strings -Wformat -Winit-self -Wmissing-include-dirs -Wno-parentheses -Wno-sign-compare -Wno-switch -IShared/include -fcolor-diagnostics -DNDEBUG -Os -flto=thin -I"/usr/local/opt/libressl/include" -I"/usr/local/include" -fvisibility=hidden -std=c++14 -fobjc-abi-version=3 -fobjc-arc -fobjc-call-cxx-cdtors  -o /Users/gplayer/build/TextMate/Frameworks/OakFoundation/src/OakFoundation.o -MMD -MF /Users/gplayer/build/TextMate/Frameworks/OakFoundation/src/OakFoundation.o.d -I/Users/gplayer/build/TextMate/include Frameworks/OakFoundation/src/OakFoundation.mm
Frameworks/OakFoundation/src/OakFoundation.mm:3:9: fatal error: 'ns/ns.h' file not found
#import <ns/ns.h>
        ^
1 error generated.
[8/9] Compile ‘Frameworks/updater/src/updater.cc’…
ninja: build stopped: subcommand failed.