Fix cmake-linux scripts deleting build dir out from under the shell

The scripts did rm -rf build && mkdir build, which invalidates the
cwd of any shell already inside the build directory. Replace with
mkdir -p build && cd build && rm -rf * to clean the contents while
preserving the directory inode.

Author: runlevel5

neo/cmake-linux-debug.sh

@@ -1,7 +1,7 @@
 rm -f idlib/precompiled.h.gch
 rm -f tools/compilers/precompiled.h.gch
 cd ..
-rm -rf build
-mkdir build
+mkdir -p build
 cd build
+rm -rf *
 cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Debug -DFFMPEG=ON -DBINKDEC=OFF -DCMAKE_CXX_COMPILER=g++ -DUSE_PRECOMPILED_HEADERS=OFF ../neo

neo/cmake-linux-release.sh

@@ -1,7 +1,7 @@
 rm -f idlib/precompiled.h.gch
 rm -f tools/compilers/precompiled.h.gch
 cd ..
-rm -rf build
-mkdir build
+mkdir -p build
 cd build
+rm -rf *
 cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DONATIVE=ON -DFFMPEG=OFF -DBINKDEC=ON ../neo

neo/cmake-linux-retail.sh

@@ -1,7 +1,7 @@
 rm -f idlib/precompiled.h.gch
 rm -f tools/compilers/precompiled.h.gch
 cd ..
-rm -rf build
-mkdir build
+mkdir -p build
 cd build
+rm -rf *
 cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DONATIVE=ON -DFFMPEG=OFF -DBINKDEC=ON -DRETAIL=ON ../neo