cmake: fix configure of Proj and PostgreSQL in standard locations

[nilason]

Fix CMake configure of Proj and PostgreSQL in standard locations. Accounting for non-existing entries in GRASS' Platform.make file.

Closes #41

[neteler]

Wow, great @nilason! 💯

mkdir build && cd build/
cmake ..
-- The C compiler identification is GNU 13.2.1
-- The CXX compiler identification is GNU 13.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/lib64/ccache/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/lib64/ccache/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found GDAL: /usr/lib64/libgdal.so (3.6.4)
-- Found GRASS: /home/mneteler/bin/grass  
-- Found GRASS_GISBASE: /home/mneteler/software/grass83/dist.x86_64-pc-linux-gnu (8.3.2dev)
-- Found PostgreSQL: /usr/include/pgsql  
-- Found CURL: /usr/lib64/libcurl.so (found version "8.0.1")  
-- Found PROJ: /usr/include  
-- Using /usr/lib/gdalplugins as GDAL shared library autoload directory
-- Found Python: /usr/bin/python3.11 (found suitable version "3.11.7", minimum required is "3.11") found components: Interpreter Development NumPy Development.Module Development.Embed 
-- Configuring done (3.7s)
-- Generating done (0.0s)
-- Build files have been written to: /home/mneteler/software/gdal-grass/build

make
[ 16%] Building CXX object CMakeFiles/gdal_grass.dir/grass.cpp.o
[ 33%] Linking CXX shared library gdal_GRASS.so
[ 33%] Built target gdal_grass
[ 50%] Building CXX object CMakeFiles/ogr_grass.dir/ogrgrassdriver.cpp.o
[ 66%] Building CXX object CMakeFiles/ogr_grass.dir/ogrgrassdatasource.cpp.o
[ 83%] Building CXX object CMakeFiles/ogr_grass.dir/ogrgrasslayer.cpp.o
[100%] Linking CXX shared library ogr_GRASS.so
[100%] Built target ogr_grass

sudo make install
[sudo] password for mneteler: 
[ 33%] Built target gdal_grass
[100%] Built target ogr_grass
Install the project...
-- Install configuration: ""
-- Installing: /usr/lib/gdalplugins/gdal_GRASS.so
-- Installing: /usr/lib/gdalplugins/ogr_GRASS.so

make test
Running tests...
Test project /home/mneteler/software/gdal-grass/build
    Start 1: autotest_ogr
1/2 Test #1: autotest_ogr .....................   Passed    1.66 sec
    Start 2: autotest_gdrivers
2/2 Test #2: autotest_gdrivers ................   Passed    1.32 sec

100% tests passed, 0 tests failed out of 2

Total Test time (real) =   2.98 sec