BuildSystem: Allow running custom version of cmake

You can now set the environment variable CMAKE_COMMAND to use custom
cmake version.

README.md

@@ -14,6 +14,8 @@ Each major component and library has its own subdirectory (kernel, userspace, li
 
 There does not exist a complete list of needed packages for building. From the top of my head I can say that *cmake*, *ninja*, *make*, *grub*, *rsync* and emulator (*qemu* or *bochs*) are needed.
 
+You need cmake version of atleast 2.26. If you are using cmake that is not found from PATH, you can set the CMAKE\_COMMAND environment variable to point to the correct cmake binary.
+
 To build the toolchain for this os. You can run the following command.
 > ***NOTE:*** The following step has to be done only once. This might take a long time since we are compiling binutils and gcc.
 ```sh

script/build.sh

@@ -14,7 +14,7 @@ make_build_dir () {
 	mkdir -p $BANAN_BUILD_DIR
 	cd $BANAN_BUILD_DIR
 	if ! [[ -f "build.ninja" ]]; then
-		cmake --toolchain=$BANAN_TOOLCHAIN_DIR/Toolchain.txt -G Ninja $BANAN_ROOT_DIR
+		$CMAKE_COMMAND --toolchain=$BANAN_TOOLCHAIN_DIR/Toolchain.txt -G Ninja $BANAN_ROOT_DIR
 	fi
 }
 
@@ -66,6 +66,10 @@ if [[ $# -eq 0 ]]; then
 	exit 1
 fi
 
+if [[ -z $CMAKE_COMMAND ]]; then
+	export CMAKE_COMMAND=cmake
+fi
+
 case $1 in
 	toolchain)
 		build_toolchain

toolchain/build.sh

@@ -153,6 +153,7 @@ build_libstdcpp () {
 }
 
 # delete everything but toolchain
+mkdir -p $BANAN_BUILD_DIR
 find $BANAN_BUILD_DIR -mindepth 1 -maxdepth 1 ! -name toolchain -exec rm -r {} +
 
 # NOTE: we have to manually create initial sysroot with libc headers