Problem while installing PostgreSQL using make

While I was trying to install PostgreSQL from the git repository to start
contributing I faced this issue. When I try to type ./configure it gives me
this error

checking for a thread-safe mkdir -p... /usr/bin/mkdir -p
checking for bison... /c/GnuWin32/bin/bison
configure: using bison (GNU Bison) 2.4.1
checking for flex... configure: error:
*** The installed version of Flex, /c/GnuWin32/bin/flex, is too old to use
with PostgreSQL.
*** Flex version 2.5.35 or later is required, but this is
C:\GnuWin32\bin\flex.exe version 2.5.4.

Look at the last two lines, the error says that the installed version of
flex is too old and is 2.4 which is correct and not too old and should be
valid but actually I can't proceed beyond this point. And I double checked
the version of flex

$ flex --version
C:\GnuWin32\bin\flex.exe version 2.5.4

and made sure that it is properly included in PATH

$ which flex
/c/GnuWin32/bin/flex