build: Fix CMake compatibility (closes merbanan#2884)

Author: zuckschwerdt

CMakeLists.txt

@@ -101,7 +101,7 @@ if("${CMAKE_C_COMPILER_ID}" MATCHES "Clang")
 endif()
 
 # Enable Static analysis on GCC13.2.0+
-if("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU" AND CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL "13.2.0")
+if("${CMAKE_C_COMPILER_ID}" STREQUAL "GNU" AND NOT "13.2.0" VERSION_GREATER CMAKE_C_COMPILER_VERSION)
     message(STATUS "Using GCC Static analysis")
     add_definitions(-fanalyzer)
 endif()

cmake/Modules/FindLibRTLSDR.cmake

@@ -29,7 +29,9 @@ set(LibRTLSDR_VERSION ${PC_LibRTLSDR_VERSION})
 include(FindPackageHandleStandardArgs)
 # handle the QUIETLY and REQUIRED arguments and set LibRTLSDR_FOUND to TRUE
 # if all listed variables are TRUE
+# Note that `FOUND_VAR LibRTLSDR_FOUND` is needed for cmake 3.2 and older.
 find_package_handle_standard_args(LibRTLSDR
+                                  FOUND_VAR LibRTLSDR_FOUND
                                   REQUIRED_VARS LibRTLSDR_LIBRARY LibRTLSDR_INCLUDE_DIR
                                   VERSION_VAR LibRTLSDR_VERSION)
 

cmake/Modules/FindLibUSB.cmake

@@ -44,7 +44,9 @@ set(LibUSB_VERSION ${PC_LibUSB_VERSION})
 include(FindPackageHandleStandardArgs)
 # handle the QUIETLY and REQUIRED arguments and set LibUSB_FOUND to TRUE
 # if all listed variables are TRUE
+# Note that `FOUND_VAR LibRTLSDR_FOUND` is needed for cmake 3.2 and older.
 find_package_handle_standard_args(LibUSB
+                                  FOUND_VAR LibUSB_FOUND
                                   REQUIRED_VARS LibUSB_LIBRARY LibUSB_INCLUDE_DIR
                                   VERSION_VAR LibUSB_VERSION)
 

cmake/Modules/GetGitRevisionDescription.cmake

@@ -111,7 +111,7 @@ function(git_describe _var)
   if(NOT GIT_FOUND AND CMAKE_HOST_WIN32)
     # fallback to VS bundled Git
     find_program(GIT_EXECUTABLE NAMES git PATH_SUFFIXES Git/cmd Git/bin)
-    find_package_handle_standard_args(Git REQUIRED_VARS GIT_EXECUTABLE)
+    find_package_handle_standard_args(Git FOUND_VAR GIT_FOUND REQUIRED_VARS GIT_EXECUTABLE)
   endif()
   if(NOT GIT_FOUND)
     set(${_var} "GIT-NOTFOUND" PARENT_SCOPE)
@@ -156,7 +156,7 @@ function(git_timestamp _var)
   if(NOT GIT_FOUND AND CMAKE_HOST_WIN32)
     # fallback to VS bundled Git
     find_program(GIT_EXECUTABLE NAMES git PATH_SUFFIXES Git/cmd Git/bin)
-    find_package_handle_standard_args(Git REQUIRED_VARS GIT_EXECUTABLE)
+    find_package_handle_standard_args(Git FOUND_VAR GIT_FOUND REQUIRED_VARS GIT_EXECUTABLE)
   endif()
   if(NOT GIT_FOUND)
     return()

docs/BUILDING.md

@@ -94,6 +94,14 @@ E.g. use:
 
     cmake -DENABLE_SOAPYSDR=ON ..
 
+::: tip
+If you use CMake older than 3.13 (check `cmake --version`), you need to build using e.g. `mkdir build ; cd build ; cmake .. && cmake --build .`
+:::
+
+::: tip
+In CMake 3.6 or older the OpenSSL search seems broken, you need to use `cmake -DENABLE_OPENSSL=NO ..`
+:::
+
 ::: warning
 If you experience trouble with SoapySDR when compiling or running: you likely mixed version 0.7 and version 0.8 headers and libs.
 Purge all SoapySDR packages and source installation from /usr/local.