Probably because reautoconf is not done yet.
Unfortunately not, doing a reautoconf and Build breaks at
gcc -DHAVE_CONFIG_H -I. -I../../../texk/web2c -I./w2c -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/mpfr/include -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/gmp/include -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/cairo/cairo -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/pixman/include -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/libpng/include -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/libs/zlib/include -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/texk/ptexenc -I/home/norbert/Development/TeX/texlive.git/Build/source/texk/ptexenc -I/home/norbert/Development/TeX/texlive.git/Build/source/Work/texk -I/home/norbert/Development/TeX/texlive.git/Build/source/texk -I../../../texk/web2c/mplibdir -Wimplicit -Wreturn-type -g -O2 -MT pmpost-pmpmathbinary.o -MD -MP -MF .deps/pmpost-pmpmathbinary.Tpo -c -o pmpost-pmpmathbinary.o `test -f 'pmpmathbinary.c' || echo '../../../texk/web2c/'`pmpmathbinary.c
In file included from ../../../texk/web2c/mplibdir/mpmathbinary.w:28:
../../../texk/web2c/mplibdir/mpmathbinary.w:44:10: fatal error: gmp/config.h: No such file or directory
#include