[GTK] CMakeLists cannot recognize gcc > 6.0.0

[tigercosmos]

In `webkit/CMakeLists.txt`, the following line seems not works well:
```
if (${CMAKE_CXX_COMPILER_VERSION} VERSION_LESS "6.0.0")
```

I have gcc 6.4.0, but it show error as:
```
$ Tools/Scripts/build-webkit --gtk -j4
Use of uninitialized value $previousContents in chomp at /home/tigercosmos/webkit/Tools/Scripts/webkitdirs.pm line 1974.
Use of uninitialized value $previousContents in string ne at /home/tigercosmos/webkit/Tools/Scripts/webkitdirs.pm line 1977.
+ /home/tigercosmos/webkit/Tools/jhbuild/jhbuild-wrapper --gtk run cmake --build /home/tigercosmos/webkit/WebKitBuild/Release --config Release -- 
[1/1] Re-running CMake...
-- The CMake build type is: Release
CMake Error at CMakeLists.txt:64 (message):
  GCC 6.0.0 is required to build WebKitGTK+, use a newer GCC version or clang
```

However, when I command the line above in `CMakeKList.txt`, it can compile well.

```
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 6.4.0-17ubuntu1~16.04' --with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs --enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --with-as=/usr/bin/x86_64-linux-gnu-as --with-ld=/usr/bin/x86_64-linux-gnu-ld --program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --with-target-system-zlib --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 6.4.0 20180424 (Ubuntu 6.4.0-17ubuntu1~16.04) 
```

[tigercosmos]

I use ` message("${CMAKE_C_COMPILER}")` to find the cc path.
It will lead to `/usr/bin/cc`

If I check the `/usr/bin/cc` version it shows `6.4.0`

However, in the cmake `CMAKE_CXX_COMPILER_VERSION` will get `5.4.0`.
So weird!

[Michael Catanzaro]

Presumably you've figured this out by now, but you probably just needed to clear your CMakeCache.txt.