failed build of SageMath on WSL

gistfile1.txt

After the First failure (below), I run `make` again and get 

```
make
make build/make/Makefile
make[1]: Entering directory `/home/walkingrandomly/github/sagemath/sage-7.5.1'
make[1]: `build/make/Makefile' is up to date.
make[1]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1'
build/bin/sage-logger \
                "cd build/make && ./install 'all'" logs/install.log
./install: fork: Cannot allocate memory
make: *** [all] Error 254
```


First failure was 

```
[gcc-5.4.0] /home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build/./gcc/xgcc -B/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build/./gcc/ -B/home/walkingrandomly/github/sagemath/sage-7.5.1/local/x86_64-unknown-linux-gnu/bin/ -B/home/walkingrandomly/github/sagemath/sage-7.5.1/local/x86_64-unknown-linux-gnu/lib/ -isystem /home/walkingrandomly/github/sagemath/sage-7.5.1/local/x86_64-unknown-linux-gnu/include -isystem /home/walkingrandomly/github/sagemath/sage-7.5.1/local/x86_64-unknown-linux-gnu/sys-include    -g -O2 -O2  -g -O2 -DIN_GCC    -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition  -isystem ./include   -fpic -mlong-double-80 -g -DIN_LIBGCC2 -fbuilding-libgcc -fno-stack-protector   -fpic -mlong-double-80 -I. -I. -I../.././gcc -I../../../src/libgcc -I../../../src/libgcc/. -I../../../src/libgcc/../gcc -I../../../src/libgcc/../include -I../../../src/libgcc/config/libbid -DENABLE_DECIMAL_BID_FORMAT -DHAVE_CC_TLS  -DUSE_TLS -o _negvdi2.o -MT _negvdi2.o -MD -MP -MF _negvdi2.dep -DL_negvdi2 -c ../../../src/libgcc/libgcc2.c -fvisibility=hidden -DHIDE_EXPORTS
[gcc-5.4.0] /home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build/./gcc/as: 32: /home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build/./gcc/as: Cannot fork
[gcc-5.4.0] make[6]: *** [_negvdi2.o] Error 1
[gcc-5.4.0] make[6]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build/x86_64-unknown-linux-gnu/libgcc'
[gcc-5.4.0] make[5]: *** [all-stage2-target-libgcc] Error 2
[gcc-5.4.0] make[5]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build'
[gcc-5.4.0] make[4]: *** [stage2-bubble] Error 2
[gcc-5.4.0] make[4]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build'
[gcc-5.4.0] make[3]: *** [all] Error 2
[gcc-5.4.0] make[3]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0/gcc-build'
[gcc-5.4.0]
[gcc-5.4.0] real        209m42.033s
[gcc-5.4.0] user        62m55.469s
[gcc-5.4.0] sys 142m55.563s
[gcc-5.4.0] ************************************************************************
[gcc-5.4.0] Error installing package gcc-5.4.0
[gcc-5.4.0] ************************************************************************
[gcc-5.4.0] Please email sage-devel (http://groups.google.com/group/sage-devel)
[gcc-5.4.0] explaining the problem and including the relevant part of the log file
[gcc-5.4.0]   /home/walkingrandomly/github/sagemath/sage-7.5.1/logs/pkgs/gcc-5.4.0.log
[gcc-5.4.0] Describe your computer, operating system, etc.
[gcc-5.4.0] If you want to try to fix the problem yourself, *don't* just cd to
[gcc-5.4.0] /home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0 and type 'make' or whatever is appropriate.
[gcc-5.4.0] Instead, the following commands setup all environment variables
[gcc-5.4.0] correctly and load a subshell for you to debug the error:
[gcc-5.4.0]   (cd '/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0' && '/home/walkingrandomly/github/sagemath/sage-7.5.1/sage' --sh)
[gcc-5.4.0] When you are done debugging, you can type "exit" to leave the subshell.
[gcc-5.4.0] ************************************************************************
make[2]: *** [/home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/lib/sage/installed/gcc-5.4.0] Error 1
make[2]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/build/make'
make[1]: *** [all-toolchain] Error 2
make[1]: Leaving directory `/home/walkingrandomly/github/sagemath/sage-7.5.1/build/make'

real    236m50.066s
user    66m10.469s
sys     395m9.953s
***************************************************************
Error building Sage.

The following package(s) may have failed to build (not necessarily
during this run of 'make all'):

* package: gcc-5.4.0
  log file: /home/walkingrandomly/github/sagemath/sage-7.5.1/logs/pkgs/gcc-5.4.0.log
  build directory: /home/walkingrandomly/github/sagemath/sage-7.5.1/local/var/tmp/sage/build/gcc-5.4.0

The build directory may contain configuration files and other potentially
helpful information. WARNING: if you now run 'make' again, the build
directory will, by default, be deleted. Set the environment variable
SAGE_KEEP_BUILT_SPKGS to 'yes' to prevent this.

make: *** [all] Error 1
```