I've always been able to compile sources from git repository https://github.com/GNOME/vala.git on my PC running "Linux Mint 18.3 Cinnamon 64-bit". I have the latest valac (0.42.4-0ubuntu1~16.04~valateam0) installed.
The last successful compile I did was from version 0.40.0.257-40b5-dirty. Today, I downloaded the latest vala.git, but when i run ./configure I get:
./configure: line 12579: syntax error near unexpected token `$flag,TEST_CFLAGS="$TEST_CFLAGS $flag"'
./configure: line 12579: ` AX_CHECK_COMPILE_FLAG($flag,TEST_CFLAGS="$TEST_CFLAGS $flag")'
Do I need to make additional environment changes? Not sure were to go from here.