git: Update Linux and macOS build instructions to use Ninja

dist/compiling/linux.md

@@ -9,7 +9,8 @@ On Linux, ImHex is built through regular GCC (or optionally Clang).
 cd ImHex
 mkdir -p build
 cd build
-CC=gcc-12 CXX=g++-12 cmake                    \
+CC=gcc-12 CXX=g++-12                          \
+cmake -G "Ninja"                              \
     -DCMAKE_BUILD_TYPE=Release                \
     -DCMAKE_INSTALL_PREFIX="/usr"             \
     -DCMAKE_C_COMPILER_LAUNCHER=ccache        \

dist/compiling/macos.md

@@ -13,10 +13,10 @@ CC=$(brew --prefix llvm)/bin/clang        \
 CXX=$(brew --prefix llvm)/bin/clang++     \
 OBJC=$(brew --prefix llvm)/bin/clang      \
 OBJCXX=$(brew --prefix llvm)/bin/clang++  \
-cmake                                     \
+cmake -G "Ninja"                          \
   -DCMAKE_BUILD_TYPE=Release              \
   -DCMAKE_INSTALL_PREFIX=./install        \
   -DIMHEX_GENERATE_PACKAGE=ON             \
   ..
-make -j4 install
+ninja install
 ```