Typos & nitpicks

Fix some typos in the READMEs.

Nit #1: Use full path when cloning repo
If we use the basename, we're forced to run the script from the directory where
we want the whole thing to live. By using the full path, we can run the script
from anywhere.

Nit #2: Use more general name for variable (generator vs ninja)

Nit #3 (noticed by Renato): Set generator explicitly in llvm-build
When ninja is not found, we use make for the build. However, we don't tell
cmake about it and instead let it choose its own default generator. This commit
explicitly sets the cmake generator to "Unix Makefiles", in case cmake's default
generator for the current platform is not make.
In order to get this to work, it was necessary to make some changes to the
safe_run helper function, so we wouldn't lose the quotes around "Unix
Makefiles".

Change-Id: I9592645d517d006b0e680a2996d498e57f009400
diff --git a/helpers/llvm-build b/helpers/llvm-build
index 8d2fab4..5b4f27d 100755
--- a/helpers/llvm-build
+++ b/helpers/llvm-build
@@ -61,10 +61,10 @@
 
 ## Choose between make and ninja
 make=make
-ninja=
+generator="Unix Makefiles"
 if which ninja 2>&1 > /dev/null; then
   make=ninja
-  ninja="-G Ninja"
+  generator="Ninja"
 fi
 
 ## Debug mode, make it lighter
@@ -83,7 +83,7 @@
 ## Re-run CMake file files are damaged / not there
 if [ ! -f build.ninja ] && [ ! -f Makefile ]; then
 	echo " + Configuring Build"
-  safe_run cmake $ninja $LLVM_SRC \
+  safe_run cmake -G "$generator" $LLVM_SRC \
             -DCMAKE_BUILD_TYPE=$build_type \
             -DLLVM_BUILD_TESTS=True \
             -DLLVM_ENABLE_ASSERTIONS=True \